The pre_category_description action is used to hook into code before a a category description is updated in the database.
A plugin (or theme) can register an action hook from the example below.
<?php
add_action( 'pre_category_description', 'do_something_before_update', 10, 2 );
function do_something_before_update( $description) {
// do something with the description before it's saved here
// the returned description is what's being saved to the database
return $description;
}
?>