What does "auxiliary" mean?

Additional senses

1. 2.(Mil.) pl. Foreign troops in the service of a nation at war; (rarely in sing.), a member of the allied or subsidiary force.
2. 3.(Gram.) 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; être and avoir, in French; avere and essere, in Italian; estar and haber, in Spanish.
3. 4.(Math.) A quantity introduced for the purpose of simplifying or facilitating some operation, as in equations or trigonometrical formulæ. Math. Dict.