WordPress at a glance
function is not described

tag_exists() WP 2.3.0

Check whether a post tag with a given name exists.

No Hooks.

Return

Mixed.

Usage

tag_exists( $tag_name );
$tag_name(int/string) (required)

Code of tag exists: wp-admin/includes/taxonomy.php WP 5.2.2

<?php
function tag_exists( $tag_name ) {
	return term_exists( $tag_name, 'post_tag' );
}