February 11, 2011 
Definition:
function activate_plugin( $plugin, $redirect = '', $network_wide = false, $silent = false ) {}
Attempts activation of plugin in a "sandbox" and redirects on success.
A plugin that is already activated will not attempt to be activated again.
Parameters
- string $plugin: Plugin path to main plugin file with plugin data.
- string $redirect: Optional. URL to redirect to.
- bool $network_wide: Whether to enable the plugin for all sites in the network or just the current site. Multisite only. Default is false.
- bool $silent: Prevent calling activation hooks. Optional, default is false.
Return values
returns:WP_Error on invalid file or null on success.
Defined actions
- activate_plugin
do_action( 'activate_plugin', $plugin, $network_wide ); - activate_’.$plugin
do_action( 'activate_' . $plugin, $network_wide ); - activated_plugin
do_action( 'activated_plugin', $plugin, $network_wide );
Source code
function activate_plugin( $plugin, $redirect = '', $network_wide = false, $silent = false ) {
$plugin = plugin_basename( trim( $plugin ) );
if ( is_multisite() && ( $network_wide || is_network_only_plugin($plugin) ) ) {
$network_wide = true;
$current = get_site_option( 'active_sitewide_plugins', array() );
} else {
$current = get_option( 'active_plugins', array() );
}
$valid = validate_plugin($plugin);
if ( is_wp_error($valid) )
return $valid;
if ( !in_array($plugin, $current) ) {
if ( !empty($redirect) )
wp_redirect(add_query_arg('_error_nonce', wp_create_nonce('plugin-activation-error_' . $plugin), $redirect)); // we'll override this later if the plugin can be included without fatal error
ob_start();
include_once(WP_PLUGIN_DIR . '/' . $plugin);
if ( ! $silent ) {
do_action( 'activate_plugin', $plugin, $network_wide );
do_action( 'activate_' . $plugin, $network_wide );
}
if ( $network_wide ) {
$current[$plugin] = time();
update_site_option( 'active_sitewide_plugins', $current );
} else {
$current[] = $plugin;
sort($current);
update_option('active_plugins', $current);
}
if ( ! $silent ) {
do_action( 'activated_plugin', $plugin, $network_wide );
}
if ( ob_get_length() > 0 ) {
$output = ob_get_clean();
return new WP_Error('unexpected_output', __('The plugin generated unexpected output.'), $output);
}
ob_end_clean();
}
return null;
}
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A HitchHackers guide through WordPress 
wicked just what I was looking for thank you
Thank you.. i was looking everywhere for this definition. Even the codex didn’t have it.