Source : Webster's Revised Unabridged Dictionary (1913)

Auxiliary \Aux*il"ia*ry\, n.; pl. {Auxiliaries}.
   1. A helper; an assistant; a confederate in some action or
      enterprise.

   2. (Mil.) pl. Foreign troops in the service of a nation at
      war; (rarely in sing.), a member of the allied or
      subsidiary force.

   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; [^e]tre and avoir, in French; avere and essere,
      in Italian; estar and haber, in Spanish.

   4. (Math.) A quantity introduced for the purpose of
      simplifying or facilitating some operation, as in
      equations or trigonometrical formul[ae]. --Math. Dict.