The Collaborative International Dictionary of English v.0.48:

Subsidy \Sub"si*dy\, n.; pl. Subsidies. [L. subsidium the
   troops stationed in reserve in the third line of battlem
   reserve, support, help, fr. subsidere to sit down, lie in
   wait: cf. F. subside. See Subside.]
   1. Support; aid; cooperation; esp., extraordinary aid in
      money rendered to the sovereign or to a friendly power.
      [1913 Webster]

            They advised the king to send speedy aids, and with
            much alacrity granted a great rate of subsidy.
                                                  --Bacon.
      [1913 Webster]

   Note: Subsidies were taxes, not immediately on on property,
         but on persons in respect of their reputed estates,
         after the nominal rate of 4s. the pound for lands, and
         2s. 8d. for goods. --Blackstone.
         [1913 Webster]

   2. Specifically: A sum of money paid by one sovereign or
      nation to another to purchase the cooperation or the
      neutrality of such sovereign or nation in war.
      [1913 Webster]

   3. A grant from the government, from a municipal corporation,
      or the like, to a private person or company to assist the
      establishment or support of an enterprise deemed
      advantageous to the public; a subvention; as, a subsidy to
      the owners of a line of ocean steamships.
      [1913 Webster]

   Syn: Tribute; grant.

   Usage: Subsidy, Tribute. A subsidy is voluntary; a
          tribute is exacted.
          [1913 Webster]

WordNet (r) 3.0 (2006):

subsidy
    n 1: a grant paid by a government to an enterprise that benefits
         the public; "a subsidy for research in artificial
         intelligence"

Moby Thesaurus II by Grady Ward, 1.0:

108 Moby Thesaurus words for "subsidy":
   TLC, accouterment, aid, alimony, allotment, allowance, annuity,
   armament, assistance, backing, bounty, bread, capitalization, care,
   carriage, carrying, catering, chandlery, contribution, daily bread,
   deficit financing, depletion allowance, dole, economic support,
   endowment, equipment, fellowship, financial assistance,
   financial backing, financial support, financing, finding,
   fitting out, funding, furnishing, furnishment, gift, grant,
   grant-in-aid, grubstake, guaranteed annual income, help,
   investment, keep, livelihood, living, logistics, maintenance,
   manna, meat, moral support, mothering, nourishment, nurture,
   old-age insurance, outfitting, patronization, paying the bills,
   pecuniary aid, pension, preparation, price support, procurement,
   providing, provision, provision of capital, provisioning,
   psychological support, public assistance, public welfare,
   purveyance, reinforcement, reliance, relief, replenishment,
   resupply, retailing, retirement benefits, reward, scholarship,
   security blanket, selling, sponsorship, stake, standing treat,
   stipend, subsidization, subsidizing, subsistence, subvention,
   supply, supplying, support, supportive relationship,
   supportive therapy, sustaining, sustainment, sustenance,
   sustentation, tax benefit, tender loving care, treat, upholding,
   upkeep, victualing, welfare, welfare aid, welfare payments



Bouvier's Law Dictionary, Revised 6th Ed (1856):

SUBSIDY, Eng. law. An aid, tax or tribute granted by parliament to the king
for the urgent occasions of the kingdom, to be levied on every subject of
ability, according to the value of his lands or goods. Jacob's Law. Dict.
h.t.
     2. The assistance given in money by one nation to another to enable it
the better to carry on a war, when such nation does not join directly in the
war, is called a subsidy. Vattel, liv. 3, Sec. 82. See Neutrality.