Giving assistance or support; helping. Acting as a subsidiary; supplementary: the main library and its auxiliary branches. Held in or used as a reserve: auxiliary troops; an auxiliary power generator. Nautical Equipped with a motor as well as sails. Grammar Of, relating to, or being an auxiliary verb. An individual or group that assists or functions in a supporting capacity: a volunteers' auxiliary at a hospital. A member of a foreign body of troops serving a country in war. Grammar An auxiliary verb. Nautical A sailing vessel equipped with a motor. Nautical A vessel, such as a supply ship or a tug, that is designed for and used in instances and services other than combat.