1. A helper; an assistant; a confederate in some action or enterprise.
2. Foreign troops in the service of a nation at war; (rarely in), a member of the allied or subsidiary force.
3. A verb which helps to form the voices, modes, and tenses of other verbs; called, also, an auxiliary verb; as, have, be, may, can, do, must, shall, and will, in English; etre and avoir, in French; avere and essere, in Italian; estar and haber, in spanish.
4. (Science: mathematics) a quantity introduced for the purpose of simplifying or facilitating some operation, as in equations or trigonometrical formulae.
Relating to something that is added but is not essential; an ancillary pump; an adjuvant discipline to forms of mysticism; The mind and emotions are auxilliary to each other.