DB_HOST not defined in WP-CONFIG

I inherited support for an already installed working WordPress with a wp-config.php file that has no definition for DB_HOST. It only has DB_NAME, DB_USER, and DB_PASSWORD defined. Does WordPress default to “localhost” even if it is not specified/defined in the wp-config file? In other words, is it definitely “localhost” if there’s no definition in this file?

Thanks
Chris