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.
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            They advised the king to send speedy aids, and with
            much alacrity granted a great rate of subsidy.
                                                  --Bacon.
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   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.
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   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.
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   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.
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   Syn: Tribute; grant.

   Usage: Subsidy, Tribute. A subsidy is voluntary; a
          tribute is exacted.
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