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}).