The Collaborative International Dictionary of English v.0.48:

Subsidiary \Sub*sid"i*a*ry\, n.; pl. Subsidiaries.
   One who, or that which, contributes aid or additional
   supplies; an assistant; an auxiliary. --Hammond.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Subsidiary \Sub*sid"i*a*ry\, a. [L. subsidiarius: cf. F.
   subsidiaire. See Subsidy.]
   1. Furnishing aid; assisting; auxiliary; helping; tributary;
      especially, aiding in an inferior position or capacity;
      as, a subsidiary stream.
      [1913 Webster]

            Chief ruler and principal head everywhere, not
            suffragant and subsidiary.            --Florio.
      [1913 Webster]

            They constituted a useful subsidiary testimony of
            another state of existence.           --Coleridge.
      [1913 Webster]

   2. Of or pertaining to a subsidy; constituting a subsidy;
      being a part of, or of the nature of, a subsidy; as,
      subsidiary payments to an ally.
      [1913 Webster]

            George the Second relied on his subsidiary treaties.
                                                  --Ld. Mahon.
      [1913 Webster]

WordNet (r) 3.0 (2006):

subsidiary
    adj 1: functioning in a supporting capacity; "the main library
           and its auxiliary branches" [syn: auxiliary,
           subsidiary, supplemental, supplementary]
    n 1: an assistant subject to the authority or control of another
         [syn: subordinate, subsidiary, underling, foot
         soldier]
    2: a company that is completely controlled by another company
       [syn: subsidiary company, subsidiary]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "subsidiary":
   accessory, accident, accidental, addendum, addition, additional,
   adjunct, adjuvant, adscititious, adventitious, ancillary,
   appendage, appurtenance, appurtenant, ascititious, assistant,
   assisting, auxiliary, casual, circumstantial, collateral,
   contingency, contingent, contributory, dotal, dower, dowered,
   dowry, endowed, extra, fortuitous, fostering, happenstance,
   helping, incidental, inessential, instrumental, invested,
   mere chance, ministerial, ministering, ministrant, minor,
   nonessential, not-self, nurtural, nutricial, other, pensionary,
   secondary, serving, stipendiary, subordinate to, subservient,
   superadded, superaddition, superfluous, supervenient, supplement,
   supplemental, supplementary, tributary, unessential