Source : Webster's Revised Unabridged Dictionary (1913)

Horsefly \Horse"fly`\, n.; pl. {Horseflies}.
   1. (Zo["o]l.) Any dipterous fly of the family {Tabanid[ae]},
      that stings horses, and sucks their blood.

   Note: Of these flies there are numerous species, both in
         Europe and America. They have a large proboscis with
         four sharp lancets for piercing the skin. Called also
         {breeze fly}. See Illust. under {Diptera}, and {Breeze
         fly}.

   2. (Zo["o]l.) The horse tick or forest fly ({Hippobosca}).

Source : WordNet®

horsefly
     n 1: winged fly parasitic on horses [syn: {horse tick}, {Hippobosca
          equina}]
     2: large swift fly the female of which sucks blood of various
        animals [syn: {cleg}, {clegg}, {horse fly}]