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Plugin API/Action Reference

This is a (hopefully) comprehensive list of action hooks available in WordPress version 2.1 and above. For more information:

(If you want to add to or clarify this documentation, please follow the style of the existing entries. Describe when the action runs, and if the action function takes arguments, describe the arguments.)

Contents

Actions Run During a Typical Request

These actions are called when a logged-in user opens the home page in Version 3.3.1. This list may show only the first time each action is called, and in many cases no function is hooked to the action. Themes and plugins can cause actions to be called multiple times and at differing times during a request. As proof of this, you can see action calls specific to the Twenty Eleven theme on this list. Cron tasks may also fire when a user visits the site, adding additional action calls. This list should be viewed as a guide line or approximation of WordPress action execution order, and not a concrete specification.

Actions are called with the function do_action(), except those marked (ref array), which are called with the function do_action_ref_array().

muplugins_loadedAfter must-use plugins are loaded
registered_taxonomyFor category, post_tag, etc.
registered_post_typeFor post, page, etc.
plugins_loadedAfter active plugins and pluggable functions are loaded
sanitize_comment_cookies
setup_theme
load_textdomainFor the default domain
after_setup_themeGenerally used to initialize theme settings/options. This is the first action hook available to themes, triggered immediately after the active theme's functions.php file is loaded. add_theme_support() should be called here, since the init action hook is too late to add some features. At this stage, the current user is not yet authenticated.
auth_cookie_malformed
auth_cookie_valid
set_current_user
initTypically used by plugins to initialize. The current user is already authenticated by this time.
└─ widgets_initUsed to register sidebars. Fired at 'init' priority 1 (and so before 'init' actions with priority ≥ 1!)
register_sidebarFor each sidebar and footer area
wp_register_sidebar_widgetFor each widget
wp_default_scripts(ref array)
wp_default_styles(ref array)
admin_bar_init
add_admin_bar_menus
wp_loadedAfter WordPress is fully loaded
parse_requestAllows manipulation of HTTP request handling (ref array)
send_headersAllows customization of HTTP headers (ref array)
parse_queryAfter query variables are set (ref array)
pre_get_postsExposes the query-variables object before a query is executed. (ref array)
posts_selection
wpAfter WP object is set up (ref array)
template_redirect
get_header
wp_enqueue_scripts
twentyeleven_enqueue_color_scheme (Specific to Twenty Eleven)
wp_head
wp_print_styles
wp_print_scripts
get_search_form
loop_start(ref array)
the_post(ref array) Allows modification of the post object immediately after query
get_template_part_contentTemplate part for the content
loop_end(ref array)
get_sidebar
dynamic_sidebar
get_search_form
pre_get_comments(ref array)
wp_meta
get_footer
get_sidebarThis 'sidebar' is a footer area
twentyeleven_credits(Specific to Twenty Eleven)
wp_footer
wp_print_footer_scripts
admin_bar_menu(ref array)
wp_before_admin_bar_render
wp_after_admin_bar_render
shutdownPHP execution is about to end

Actions Run During an Admin Page Request

These actions are run when a logged-in user opens the Posts page in Version 3.3.1. This list shows only the first time an action is called, and in many cases no function is hooked to the action. Each admin page has a different list of actions depending upon the purpose of the page and the plugins installed. This list should be viewed as a guide line or approximation, and not a concrete specification.

In these actions, (hookname) depends on the page. For the Posts page it is edit.php, or for a theme's Background page it is appearance_page_custom-background.

Actions are called with the function do_action(), except those marked (ref array), which are called with the function do_action_ref_array().

muplugins_loadedAfter must-use plugins are loaded
registered_taxonomyFor category, post_tag, etc.
registered_post_typeFor post, page, etc.
plugins_loadedAfter active plugins and pluggable functions are loaded
sanitize_comment_cookies
setup_theme
load_textdomainFor domain default
after_setup_themeAt this stage, the current user is not yet authenticated.
load_textdomainFor domain twentyeleven
auth_cookie_valid
set_current_user
initTypically used by plugins to initialize. The current user is already authenticated by this time.
└─ widgets_initUsed to register sidebars. This is fired at 'init', with a priority of 1.
register_sidebarFor each sidebar
wp_register_sidebar_widgetFor each widget
wp_default_scripts(ref array)
wp_default_styles(ref array)
admin_bar_init
add_admin_bar_menus
wp_loadedAfter WordPress is fully loaded
auth_cookie_valid
auth_redirect
_admin_menuSee also: _user_admin_menu, _network_admin_menu
admin_menuSee also: user_admin_menu, network_admin_menu
admin_init
current_screen
load-(page)
send_headersWhere custom HTTP headers can be added
pre_get_postsExposes the query-variables object before a query is executed. (ref array)
posts_selection
wpAfter WP object is set up (ref array)
admin_xml_ns
admin_xml_ns
admin_enqueue_scripts
admin_print_styles-(hookname)
admin_print_styles
admin_print_scripts-(hookname)
admin_print_scripts
wp_print_scripts
admin_head-(hookname)
admin_head
adminmenu
in_admin_header
admin_notices
all_admin_notices
(hookname)
restrict_manage_posts
the_post(ref array)
pre_user_query(ref array)
in_admin_footer
admin_footer
admin_bar_menu(ref array)
wp_before_admin_bar_render
wp_after_admin_bar_render
admin_print_footer_scripts
admin_footer-(hookname)Admin page footer
shutdownPHP execution is about to end
wp_dashboard_setupAllows customization of admin Dashboard

Post, Page, Attachment, and Category Actions (Admin)

post_submitbox_misc_actions 
Runs when an editing page gets generated to add some content (eg. fields) to the submit box (where the publish button is shown). No function arguments.
add_attachment 
Runs when an attached file is first added to the database. Action function arguments: attachment ID.
add_category 
Same as create_category.
category_add_form_fields 
Runs when category add form is cerated in admin. Useful to add a field in this form before the submit button
category_edit_form 
Runs when category edit form is created in admin. Useful to add a new field to this form
clean_post_cache 
Runs when post cache is cleaned. Action function arguments: post ID. See clean_post_cache().
create_category 
Runs when a new category is created. Action function arguments: category ID.
delete_attachment 
Runs just before an attached file is deleted from the database. Action function arguments: attachment ID. (Prior to version 2.8 this hook was triggered after attachment was deleted.)
delete_category 
Runs just after a category is deleted from the database and its corresponding links/posts are updated to remove the category. Action function arguments: category ID.
wp_trash_post 
Runs when a post or page is about to be trashed. Action function arguments: post or page ID.
trashed_post 
Runs just after a post or page is trashed. Action function arguments: post or page ID.
untrash_post 
Runs just before undeletion, when a post or page is restored. Action function arguments: post or page ID.
untrashed_post 
Runs just after undeletion, when a post or page is restored. Action function arguments: post or page ID.
before_delete_post 
Runs when a post or page is about to be deleted. Comments, attachments and metadata are still available. Action function arguments: post or page ID.
delete_post 
Runs when a post or page is about to be deleted. Comments, attachments and metadata are already deleted. Action function arguments: post or page ID.
deleted_post 
Runs just after a post or page is deleted. Action function arguments: post or page ID.
edit_attachment 
Runs when an attached file is edited/updated to the database. Action function arguments: attachment ID.
edit_category 
Runs when a category is updated/edited, including when a post or blogroll link is added/deleted or its categories are updated (which causes the count for the category to update). Action function arguments: category ID.
edit_post 
Runs when a post or page is updated/edited, including when a comment is added or updated (which causes the comment count for the post to update). Action function arguments: post or page ID.
pre_post_update 
Runs just before a post or page is updated. Action function arguments: post or page ID.
post_updated 
Runs after a post or page is updated. Action function arguments: post or page ID, WP_Post object of the post before the update and after the update.
transition_post_status  
Runs when any post status transition occurs. Action function arguments: $old_status, $new_status, $post object. (See also Post Status Transitions.)
(old status)_to_(new status) 
Runs when a post changes status from $old_status to $new_status. Action function arguments: $post object. (See also Post Status Transitions.)
(status)_(post_type) 
Runs when a post of type $post_type is transitioned to $status from any other status. Action function arguments: post ID, $post object. (See also Post Status Transitions.)
publish_post (not deprecated) 
Runs when a post is published, or if it is edited and its status is changed to "published". This action hook conforms to the (status)_(post_type) action hook type. Action function arguments: post ID, $post object. (See also Post Status Transitions.)
publish_page 
Runs when a page is published, or if it is edited and its status is changed to "published". This action hook conforms to the (status)_(post_type) action hook type. Action function arguments: post ID, $post object. (See also Post Status Transitions.)
publish_phone 
Runs just after a post is added via email. Action function argument: post ID.
publish_future_post 
Runs when a future post or page is published. Action function arguments: post ID.
save_post 
Runs whenever a post or page is created or updated, which could be from an import, post/page edit form, xmlrpc, or post by email. Action function arguments: post ID and post object. Runs after the data is saved to the database. Note that post ID may reference a post revision and not the last saved post. Use wp_is_post_revision() to get the ID of the real post.
updated_postmeta 
Runs when a metadata has been updated.
wp_insert_post 
Same as save_post, runs immediately afterwards.
xmlrpc_publish_post 
Runs when a post is published via XMLRPC request, or if it is edited via XMLRPC and its status is "published". Action function arguments: post ID.

Comment, Ping, and Trackback Actions

comment_closed 
Runs when the post is marked as not allowing comments while trying to display comment entry form. Action function argument: post ID.
comment_id_not_found 
Runs when the post ID is not found while trying to display comments or comment entry form. Action function argument: post ID.
comment_flood_trigger 
Runs when a comment flood is detected, just before wp_die is called to stop the comment from being accepted. Action function arguments: time of previous comment, time of current comment.
comment_(old status)_to_(new status) 
Runs when a comment status transition occurs. Action function arguments: Comment object.
comment_on_draft 
Runs when the post is a draft while trying to display a comment entry form or comments. Action function argument: post ID.
comment_post 
Runs just after a comment is saved in the database. Action function arguments: comment ID, approval status ("spam", or 0/1 for disapproved/approved).
edit_comment 
Runs after a comment is updated/edited in the database. Action function arguments: comment ID.
delete_comment 
Runs just before a comment is deleted. Action function arguments: comment ID.
deleted_comment 
Runs just after a comment is deleted. Action function arguments: comment ID.
trash_comment 
Runs just before a comment is trashed. Action function arguments: comment ID.
trashed_comment 
Runs just after a comment is trashed. Action function arguments: comment ID.
comment_closed 
Runs when the post is marked as not a spam.
pingback_post 
Runs when a ping is added to a post. Action function argument: comment ID.
pre_ping 
Runs before a ping is fully processed. Action function arguments: array of the post links to be processed, and the "pung" setting for the post.
trackback_post 
Runs when a trackback is added to a post. Action function argument: comment ID.
wp_blacklist_check 
Runs to check whether a comment should be blacklisted. Action function arguments: author name, author email, author URL, comment text, author IP address, author's user agent (browser). Your function can execute a wp_die to reject the comment, or perhaps modify one of the input arguments so that it will contain one of the blacklist keywords set in the WordPress options.
wp_insert_comment 
Runs whenever a comment is created.
wp_set_comment_status 
Runs when the status of a comment changes. Action function arguments: comment ID, status string indicating the new status ("delete", "approve", "spam", "hold").

Blogroll Actions

add_link 
Runs when a new blogroll link is first added to the database. Action function arguments: link ID.
delete_link 
Runs when a blogroll link is deleted. Action function arguments: link ID.
edit_link 
Runs when a blogroll link is edited. Action function arguments: link ID.

Feed Actions

atom_entry 
Runs just after the entry information has been printed (but before closing the entry tag) for each blog entry in an atom feed.
atom_head 
Runs just after the blog information has been printed in an atom feed, just before the first entry.
atom_ns 
Runs inside the root XML element for an atom feed (to add namespaces).
commentrss2_item 
Runs just after a single comment's information has been printed in a comment feed (but before closing the item tag). Action function arguments: comment ID, post ID.
do_feed_(feed) 
Runs when a feed is generated, where feed is the type of feed (rss2, atom, rdf, etc.). Use priority less than 10 to run before printing the feed. Action function argument: true (the feed is for comments) or false (it is for posts).
rdf_header 
Runs just after the blog information has been printed in an RDF feed, just before the first entry.
rdf_item 
Runs just after the entry information has been printed (but before closing the item tag) for each blog entry in an RDF feed.
rdf_ns 
Runs inside the root XML element in an RDF feed (to add namespaces).
rss_head 
Runs just after the blog information has been printed in an RSS feed, just before the first entry.
rss_item 
Runs just after the entry information has been printed (but before closing the item tag) for each blog entry in an RSS feed.
rss2_head 
Runs just after the blog information has been printed in an RSS 2 feed, just before the first entry.
rss2_item 
Runs just after the entry information has been printed (but before closing the item tag) for each blog entry in an RSS 2 feed.
rss2_ns 
Runs inside the root XML element in an RSS 2 feed (to add namespaces).

Template Actions

after_setup_theme 
Runs during a themes initialization. Is generally used to perform basic setup, registration, and init actions for a theme.
comment_form 
Runs at the bottom of a comment form rendered by comment_form(), right before the closing </form>. Action function argument: the post ID.
do_robots 
Runs when the template file chooser determines that it is a robots.txt request.
do_robotstxt 
Runs in the do_robots() function before it prints out the Disallow lists for the robots.txt file.
get_footer 
Runs when the template calls the get_footer() function, just before the footer.php template file is loaded.
get_header 
Runs when the template calls the get_header() function, just before the header.php template file is loaded.
switch_theme 
Runs when the blog's theme is changed. Action function argument: name of the new theme. If used in a theme, it only works if the theme that adds action is the one being disabled.
after_switch_theme 
Runs when the blog's theme is changed. Action function argument: name of the new theme. If used in a theme, it only works if the theme that adds action is the one being enabled. Can be used to run certain code when enabling a theme.
load-themes.php 
Runs when the theme is activate or deactivate (replace by an other).
template_redirect 
Runs before the determination of the template file to be used to display the requested page.
wp_footer 
Runs when the template calls the wp_footer() function, generally near the bottom of the blog page.
wp_head 
Runs when the template calls the wp_head() function. This hook is generally placed near the top of a page template between <head> and </head>. This hook does not take any parameters.
wp_meta 
Runs when the sidebar.php template file calls the wp_meta() function, to allow the plugin to insert content into the sidebar.
wp_print_scripts 
Runs just before WordPress prints registered JavaScript scripts into the page header.

Administrative Actions

activate_(plugin file name) 
Runs when the plugin is first activated. See Function_Reference/register_activation_hook.
activity_box_end 
Runs at the end of the activity box on the admin Dashboard screen.
add_category_form_pre 
Runs before the add category form is put on the screen in the admin menus.
add_option_(option_name) 
Runs after a WordPress option has been added by the add_option() function. Action function arguments: option name, option value. You must add an action for the specific options that you want to respond to, such as 'add_option_foo' to respond when option "foo" has been added.
add_option 
Runs before an option gets added to the database.
added_option 
Runs after an an option has been added.
admin_head 
Runs in the HTML <head> section of the admin panel.
admin_head-(page_hook) or admin_head-(plugin_page) 
Runs in the HTML <head> section of a specific admin page or the admin panel of a plugin-generated page.
admin_init 
Runs at the beginning of every admin page before the page is rendered. See wp-admin/admin.php, wp-admin/admin-post.php, and wp-admin/admin-ajax.php.
admin_footer-(plugin_page) 
Runs at the end of the <body> section of the admin panel of a plugin-generated page.
admin_post_(action) 
also: admin_post_nopriv_(action) - Runs a handler for an unspecified GET or POST request.
admin_footer 
Runs at the end of the admin panel inside the body tag
admin_enqueue_scripts 
Runs in the HTML header so a plugin or theme can enqueue JavaScript and CSS to all admin pages.
admin_print_scripts 
Runs in the HTML header so a plugin can add JavaScript scripts to all admin pages.
admin_print_scripts-(page_hook) or admin_print_scripts-(plugin_page) 
Runs to print JavaScript scripts in the HTML head section of a specific plugin-generated admin page. The (page_hook) is returned when using any of the functions that add plugin menu items to the admin menu: add_management_page(), add_options_page(), etc. Example:
function myplugin_menu() {
  if ( function_exists('add_management_page') ) {
    $page = add_management_page( 'myplugin', 'myplugin', 'manage_options', 'myplugin_slug', 'myplugin_admin_page' );
    add_action( "admin_print_scripts-$page", 'myplugin_admin_head' );
  }
} 
admin_print_styles 
Runs in the HTML header so a plugin can add CSS/Stylesheets to all admin pages.
admin_print_styles-(page_hook) or admin_print_style-(plugin_page) 
Runs when styles should be enqueued with wp_enqueue_style() for a particular admin page. Use the return value of a function such as add_submenu_page() to determine the value of (page_hook).
check_passwords 
Runs to validate the double-entry of password when creating a new user. Action function arguments: array of login name, first password, second password.
dbx_page_advanced 
Runs at the bottom of the "advanced" section on the page editing screen in the admin menus.
dbx_page_sidebar 
Runs at the bottom of the sidebar on the page editing screen in the admin menus.
dbx_post_advanced 
Runs at the bottom of the "advanced" section on the post editing screen in the admin menus.
dbx_post_sidebar 
Runs at the bottom of the sidebar on the post editing screen in the admin menus. Use add_meta_box() in Wordpress 2.5 and higher.
deactivate_(plugin file name) 
Runs when a plugin is deactivated.
delete_option_(option_name) 
Runs after a WordPress option has been deleted by the delete_option() function. Action function arguments: option name. You must add an action for the specific options that you want to respond to, such as 'delete_option_foo' to respond when option "foo" has been deleted.
delete_option 
Runs before an option gets deleted from the database.
deleted_option 
Runs after an an option has been deleted.
delete_user 
Runs when a user is deleted. Action function arguments: user ID.
edit_category_form 
Runs after the add/edit category form is put on the screen (but before the end of the HTML form tag).
edit_category_form_pre 
Runs before the edit category form is put on the screen in the admin menus.
edit_tag_form 
Runs after the add/edit tag form is put on the screen (but before the end of the HTML form tag).
edit_tag_form_pre 
Runs before the edit tag form is put on the screen in the admin menus.
edit_form_top 
Runs inside the form before the title on WordPress post edit screen (and Custom Post Types), but after the inital hidden fields (user_ID, action, etc.).
edit_form_after_title 
Runs after the title on WordPress post edit screen (and Custom Post Types) but before the built in WordPress content area.
edit_form_after_editor 
Runs just after the WordPress post editor but before all other meta boxes. also available in Custom Post Types.
edit_form_advanced 
Runs just before the "advanced" section of the post editing form in the admin menus.
edit_page_form 
Runs just before the "advanced" section of the page editing form in the admin menus.
edit_user_profile 
Runs near the end of the user profile editing screen in the admin menus.
load-(page) 
Runs when an administration menu page is loaded. This action is not usually added directly -- see Adding Administration Menus for more details of how to add admin menus. If you do want to use it directly, the return value from add_options_page() and similar functions gives you the "(page)" part of the action name.
login_form 
Runs just before the end of the login form.
login_head 
Runs just before the end of the HTML head section of the login page.
lost_password 
Runs before the "retrieve your password by email" form is printed on the login screen.
lostpassword_form 
Runs at the end of the form used to retrieve a user's password by email, to allow a plugin to supply extra fields.
lostpassword_post 
runs when the user has requested an email message to retrieve their password, to allow a plugin to modify the PHP $_POST variable before processing.
manage_link_custom_column 
Runs when there is an unknown column name for the blogroll managing admin screen. Action function arguments: column name, link ID. See also filter manage_link_columns in the Plugin API/Filter Reference, which adds custom columns.
manage_posts_custom_column 
Runs when there is an unknown column name for the managing posts admin screen. Action function arguments: column name, post ID. See also filter manage_posts_columns in the Plugin API/Filter Reference, which adds custom columns. (see Scompt's tutorial for examples and use.)
manage_pages_custom_column 
Runs when there is an unknown column name for the managing pages admin screen. Action function arguments: column name, page ID. See also filter manage_pages_columns in the Plugin API/Filter Reference, which adds custom columns.
manage_media_custom_column 
Runs when there is an unknown column name for the managing media admin screen. Action function arguments: column name, page ID. See also filter manage_media_columns in the Plugin API/Filter Reference, which adds custom columns.
manage_{$post_type}_posts_custom_column 
Runs when there is an unknown column name for the managing custom post type admin screen. Action function arguments: column name, post ID. See also filter manage_${post_type}_posts_columns in the Plugin API/Filter Reference, which adds custom columns for custom post types.
password_reset 
Runs before the user's password is reset to a random new password.
personal_options_update 
Runs when a user updates personal options from the admin screen.
plugins_loaded 
Runs after all plugins have been loaded.
profile_personal_options 
Runs at the end of the Personal Options section of the user profile editing screen.
profile_update 
Runs when a user's profile is updated. Action function argument: user ID.
quick_edit_custom_box 
Runs when there is an unknown column name when creating the quick editor.
register_form 
Runs just before the end of the new user registration form.
register_post 
Runs before a new user registration request is processed.
restrict_manage_posts 
Runs before the list of posts to edit is put on the screen in the admin menus.
retrieve_password 
Runs when a user's password is retrieved, to send them a reminder email. Action function argument: login name.
set_current_user 
Runs after the user has been changed by the default wp_set_current_user() function. Note that wp_set_current_user() is also a "pluggable" function, meaning that plugins can override it; see Plugin API).
show_user_profile 
Runs near the end of the user profile editing screen.
sidebar_admin_page 
Runs after the main content on the widgets admin page.
sidebar_admin_setup 
Runs early when editing the widgets displayed in sidebars.
simple_edit_form 
Runs at the end of the "simple" post editing form in the admin menus (by default the simple form is used only for bookmarklets -- it doesn't have the "advanced" sections).
update_option_(option_name) 
Runs after a WordPress option has been updated by the update_option() function. Action function arguments: old option value, new option value. You must add an action for the specific options that you want to respond to, such as 'update_option_foo' to respond when option "foo" has been updated.
update_option 
Runs before an option gets updated to the database.
updated_option 
Runs after an option has been updated.
upload_files_(tab) 
Runs to print a screen on the upload files admin screen; "tab" is the name of the custom action tab. Define custom tabs using the wp_upload_tabs filter (see Plugin API/Filter Reference).
user_new_form 
Runs near the end of the "Add New" user screen. Action function argument: Passes the string "add-existing-user" on multisite or "add-new-user" on single site and for network admins.
user_profile_update_errors 
Runs just before updated user details are committed to the database.
wpmu_new_user 
Runs when a user's profile is first created in a Multisite environment. Action function argument: user ID. If not in Multisite then use user_register.
user_register 
Runs when a user's profile is first created. Action function argument: user ID.
welcome_panel 
Allows you to hide the Welcome Panel in the Dashboard. This is also a smart filter, which hides the related Screen Option.
wp_ajax_(action) 
also: wp_ajax_nopriv_(action) - Runs to do an unknown type of AJAX request handler.
wp_authenticate 
Runs to authenticate a user when they log in. Action function arguments: array with user name and password.
wp_login 
Runs when a user logs in.
wp_logout 
Runs when a user logs out.

Dashboard "Right Now" Widget Actions

right_now_content_table_end 
Adds table rows at the bottom the content column of the Right Now Dashboard widget.
right_now_table_end 
Called after displaying the number of Spam comments in the Discussion column of the Right Now Dashboard widget.
right_now_discussion_table_end 
Called after displaying the number of Spam comments and after the right_now_table_end action in the Discussion column of the Right Now Dashboard widget.
right_now_end 
Called after the current version information is displayed on the Right Now Dashboardwidget. (Note: In v3.4, this is actually rightnow_end. See ticket #21046.)
activity_box_end 
Last action called on the Right Now Dashboard widget.

Advanced Actions

This section contains actions related to the queries WordPress uses to figure out what posts to display, the WordPress loop, activating plugins, and other fundamental-level WordPress code.

activated_plugin 
Runs any time any plugin is successfully activated
add_meta_boxes 
Runs when "edit post" page loads. (3.0+)
admin_menu 
Runs after the basic admin panel menu structure is in place.
network_admin_notices 
Runs after the admin menu is printed to network admin screens.
user_admin_notices 
Runs after the admin menu is printed to user admin screens.
admin_notices 
Runs after the admin menu is printed to screens that aren't network- or user-admin screens.
all_admin_notices 
Runs after the admin menu is printed to all screens.
blog_privacy_selector 
Runs after the default blog privacy options are printed on the screen.
check_admin_referer 
Runs in the default check_admin_referrer() function after the nonce has been checked for security purposes, to allow a plugin to force WordPress to die for extra security reasons. Note that check_admin_referrer is also a "pluggable" function, meaning that plugins can override it; see Plugin API).
check_ajax_referer 
Runs in the default check_ajax_referer() function (which is called when an AJAX request goes to the wp-admin/admin-ajax.php script) after the user's login and password have been successfully validated from cookies, to allow a plugin to force WordPress to die for extra security reasons. Note that check_ajax_referer is also a "pluggable" function, meaning that plugins can override it; see Plugin API).
customize_register 
Runs on every request, allowing developers to register new theme options and controls for use with the Theme Customization API (as of Version 3.4).
deactivated_plugin 
Runs any time any plugin is successfully de-activated
generate_rewrite_rules 
Runs after the rewrite rules are generated. Action function arguments: WP_Rewrite object ($wp_rewrite) by reference. Note that it is easier to use the rewrite_rules_array filter instead of this action, to modify the rewrite rules.
init 
Runs after WordPress has finished loading but before any headers are sent. Useful for intercepting $_GET or $_POST triggers.
loop_end 
Runs after the last post of the WordPress loop is processed.
loop_start 
Runs before the first post of the WordPress loop is processed.
network_admin_menu 
Runs when the basic menu structure is prepared for the Network administration page. (Administration Menus)
parse_query 
Runs at the end of query parsing in the main query or any instance of WP_Query, such as query_posts, get_posts, or get_children. Action function arguments: WP_Query object by reference.
parse_request 
Runs after the query request is parsed inside the main WordPress function wp. Action function argument: WP object ($wp) by reference.
pre_get_posts 
Runs before a query is executed in the main query or any instance of WP_Query, such as query_posts(), get_posts(), or get_children(). This hook is called after the query variable object is created, but before the query is actually run, and can be used to to alter the primary query before it is run. Also see is_main_query(). Action function arguments: WP_Query object by reference.
sanitize_comment_cookies 
Runs after cookies have been read from the HTTP request.
send_headers 
Runs after the basic HTTP headers are sent inside the main WordPress function wp(). Action function argument: WP object ($wp) by reference.
shutdown 
Runs when the page output is complete.
update_(meta_type)_meta 
Runs when a metadata gets saved.
updated_(meta_type)_meta 
Runs when a metadata has been updated.
upgrader_process_complete 
Runs when the plugin downloader/upgrader class finishes running
wp_loaded 
This hook is fired once WP, all plugins, and the theme are fully loaded and instantiated.
wp 
Executes after the query has been parsed and post(s) loaded, but before any template execution, inside the main WordPress function wp(). Useful if you need to have access to post data but can't use templates for output. Action function argument: WP object ($wp) by reference.

Further Reading