The edit_terms action is used to hook into code before a term is updated in the database.
A plugin (or theme) can register an action hook from the example below.
<?php
add_action( 'edit_terms', 'do_something_or_stop_update', 10, 2 );
function do_something_or_stop_update( $term_id, $taxonomy ){
//do something before update OR
//redirect back to the edit page to disable updates (remember to exit)
}
?>