The Collaborative International Dictionary of English v.0.48:

Deer \Deer\ (d[=e]r), n. sing. & pl. [OE. der, deor, animal,
   wild animal, AS. de['o]r; akin to D. dier, OFries. diar, G.
   thier, tier, Icel. d[=y]r, Dan. dyr, Sw. djur, Goth. dius; of
   unknown origin. [root]71.]
   1. Any animal; especially, a wild animal. [Obs.] --Chaucer.
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            Mice and rats, and such small deer.   --Shak.
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            The camel, that great deer.           --Lindisfarne
                                                  MS.
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   2. (Zool.) A ruminant of the genus Cervus, of many species,
      and of related genera of the family Cervid[ae]. The
      males, and in some species the females, have solid
      antlers, often much branched, which are shed annually.
      Their flesh, for which they are hunted, is called
      venison.
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   Note: The deer hunted in England is Cervus elaphus, called
         also stag or red deer; the fallow deer is Cervus
         dama; the common American deer is Cervus
         Virginianus; the blacktailed deer of Western North
         America is Cervus Columbianus; and the mule deer of
         the same region is Cervus macrotis. See Axis,
         Fallow deer, Mule deer, Reindeer.
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   Note: Deer is much used adjectively, or as the first part of
         a compound; as, deerkiller, deerslayer, deerslaying,
         deer hunting, deer stealing, deerlike, etc.
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   Deer mouse (Zool.), the white-footed mouse (Peromyscus
      leucopus, formerly Hesperomys leucopus) of America.

   Small deer, petty game, not worth pursuing; -- used
      metaphorically. (See citation from Shakespeare under the
      first definition, above.) "Minor critics . . . can find
      leisure for the chase of such small deer." --G. P. Marsh.
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WordNet (r) 3.0 (2006):

red deer
    n 1: common deer of temperate Europe and Asia [syn: red deer,
         elk, American elk, wapiti, Cervus elaphus]