The Collaborative International Dictionary of English v.0.48:

Adjuvant \Ad"ju*vant\, a. [L. adjuvans, p. pr. of adjuvare to
   aid: cf. F. adjuvant. See Aid.]
   Helping; helpful; assisting. [R.] "Adjuvant causes."
   --Howell.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Adjuvant \Ad"ju*vant\ ([a^]d"j[-u]*vant), n. [L. adjuvamentum,
   fr. adjuvare to help; ad + juvare to help.]
   1. An assistant. [R.] --Yelverton. AS
      [1913 Webster]

   2. (Med.) An ingredient in a prescription drug, which aids or
      modifies the action of the principal ingredient. AS
      [1913 Webster]

   3. (Med., Immunology) A substance that non-specifically
      enhances immune response to an antigen, such as by
      enhancing the production of antibodies. --AS
      [AS]

WordNet (r) 3.0 (2006):

adjuvant
    adj 1: 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]
    2: enhancing the action of a medical treatment; "the adjuvant
       action of certain bacteria"
    n 1: an additive that enhances the effectiveness of medical
         treatment

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "adjuvant":
   accession, accessory, accompaniment, addenda, addendum, additament,
   addition, additive, additory, additum, adjunct, alterative,
   analeptic, ancillary, annex, annexation, appanage, appendage,
   appendant, appurtenance, appurtenant, assistant, assisting,
   attachment, augment, augmentation, auxiliary, carminative, coda,
   collateral, complement, concomitant, continuation, contributory,
   corollary, corrective, counterirritant, curative, emmenagogue,
   expectorant, extension, extrapolation, fixture, fostering, healing,
   helping, hormone, iatric, increase, increment, instrumental,
   maturative, medicative, medicinal, ministerial, ministering,
   ministrant, nurtural, nutricial, offshoot, pendant, reinforcement,
   remedial, restorative, sanative, sanatory, serving, side effect,
   side issue, subservient, subsidiary, supplement, synergistic,
   tailpiece, therapeutic, theriac, undergirding, vasodilator,
   vitamin