A ruminant mammal with antlers and hooves of the family Cervidae, or one of several similar animals from related families of the order Artiodactyla
(in particular) one of the smaller animals of this family, distinguished from a moose or elk
I wrecked my car after a deer ran across the road.
The meat of such an animal; venison.
Oh, I've never had deer before.
(except in the phrase "small deer") an animal, especially a quadrupedal mammal, as opposed to a bird, fish, etc.