Real Dictionary

deer
countable noun: A deer is a large wild animal that eats grass and leaves. A male deer usually has large, branching horns.

deer in American English
any of a family (Cervidae) of ruminants, including the elk, moose, and reindeer; esp., the smaller species, as the white-tailed deer and mule deer: in most species, usually only the males grow and shed bony antlers annually
noun: any of several ruminants of the family Cervidae, most of the males of which have solid, deciduous antlers

deer in British English
noun: any ruminant artiodactyl mammal of the family Cervidae, including reindeer, elk, muntjacs, and roe deer, typically having antlers in the male