A deer is a ruminant mammal belonging to the family Cervidae. A number of broadly similar animals from related families within the order Artiodactyla are often also called deer.

Depending on their species, male deer are called stags, harts, bucks or bulls, and females are called hinds, does or cows. Young deer are called fawns or calves. Hart, from Old English heorot ‘deer’, is a term for a stag, particularly a Red Deer stag past its fifth year. It is not commonly used, but Shakespeare makes several references, punning the sound alike "hart" and "heart" for example in Twelfth Night. "The White Hart" and "The Red Hart" are common English pub names, and the county Hertfordshire is named after them.

The history of the word deer is quite interesting in that it was originally quite broad in meaning and came to be specialized. In Middle English, der (O.E. dēor) meant a beast or animal of any kind. [1] This general sense gave way to the modern sense by the end of the Middle English period, around 1500. The German word tier, the cognate of English deer, still has the general sense of "animal." The adjective of relation pertaining to deer is cervine.