The Collaborative International Dictionary of English v.0.48:

Auxiliary \Aux*il"ia*ry\ (?; 106), a. [L. auxiliarius, fr.
   auxilium help, aid, fr. augere to increase.]
   Conferring aid or help; helping; aiding; assisting;
   subsidiary; as auxiliary troops.
   [1913 Webster]

   Auxiliary scales (Mus.), the scales of relative or
      attendant keys. See under Attendant, a.

   Auxiliary verbs (Gram.). See Auxiliary, n., 3.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Auxiliary \Aux*il"ia*ry\, n.; pl. Auxiliaries.
   1. A helper; an assistant; a confederate in some action or
      enterprise.
      [1913 Webster]

   2. (Mil.) pl. Foreign troops in the service of a nation at
      war; (rarely in sing.), a member of the allied or
      subsidiary force.
      [1913 Webster]

   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.
      [1913 Webster]

   4. (Math.) A quantity introduced for the purpose of
      simplifying or facilitating some operation, as in
      equations or trigonometrical formul[ae]. --Math. Dict.
      [1913 Webster]

WordNet (r) 3.0 (2006):

auxiliary
    adj 1: functioning in a supporting capacity; "the main library
           and its auxiliary branches" [syn: auxiliary,
           subsidiary, supplemental, supplementary]
    2: furnishing added support; "an ancillary pump"; "an adjuvant
       discipline to forms of mysticism"; "The mind and emotions are
       auxiliary to each other" [syn: accessory, adjunct,
       ancillary, adjuvant, appurtenant, auxiliary]
    n 1: someone who acts as assistant [syn: aide, auxiliary]