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]

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "auxiliary":
   abetting, accessory, accident, accidental, acolyte, addendum,
   addition, additional, adjunct, adjutant, adjuvant, adscititious,
   adventitious, agent, aid, aide, aide-de-camp, aider, aiding,
   alter ego, ancillary, another, appendage, appurtenance,
   appurtenant, ascititious, assistance, assistant, assisting,
   attendant, auxiliary verb, backing, best man, casual,
   circumstantial, coadjutant, coadjutor, coadjutress, coadjutrix,
   collateral, complementary, contingency, contingent, contributory,
   copula, defective verb, deponent verb, deputy, executive officer,
   extra, farther, finite verb, fortuitous, fostering, fresh, further,
   girl Friday, happenstance, help, helper, helpful, helping,
   helpmate, helpmeet, impersonal verb, incidental, inessential,
   infinitive, instrumental, intransitive, intransitive verb,
   lieutenant, linking verb, man Friday, mere chance, ministerial,
   ministering, ministrant, modal auxiliary, more, neuter verb, new,
   nonessential, not-self, nurtural, nutricial, other, paranymph,
   paraprofessional, peripheral, plus, reserve, second, secondary,
   servant, serving, sideman, spare, subordinate, subservient,
   subsidiary, superadded, superaddition, superfluous, supernumerary,
   supervenient, supplement, supplemental, supplementary, support,
   supporter, supporting, supporting actor,
   supporting instrumentalist, supportive, surplus, transitive,
   tributary, ulterior, unessential, upholding, verb, verb phrase,
   verbal