Customize private posts prefix in WordPress. Source: http://www.wpbeginner.com/wp-tutorials/how-to-change-private-and-protected-posts-prefix-in-wordpress/

customize-private-posts-prefix.php

<?php

//* Customize private posts prefix in WordPress
function prefix_customize_private_posts_prefix() {
    return 'Editors Only: %s';
}
add_filter('private_title_format', 'prefix_customize_private_posts_prefix');