Musk deer can refer to any one or all of the seven species that make up Moschus, which the only genus of family Moschidae. Musk deer are more primitive than cervids, or true deer, because they lack antlers and facial glands, and possess only a single pair of teats, a gall bladder, a caudal gland, a pair of tusk-like teeth and—of particular economic importance to humans—a musk gland. As is implied by their classification under the order Artiodactyla, they are artiodactyls.

Musk deer live mainly in forested and alpine scrub habitats in the mountains of southern Asia, notably the Himalayas. Moschids, the proper term when referring to this type of deer rather than one/multiple species of musk deer, are entirely Asian in their present distribution, being extinct in Europe where the earliest musk deer are known to have existed from Oligocene deposits.

Musk deer resemble small deer with a stocky build, and hind legs longer than their front legs. They are approximately 80 to 100 centimetres (31 to 39 in) in length, 50 to 70 centimetres (20 to 28 in) tall at the shoulder, and weigh between 7 and 17 kilograms (15 and 37 lb). The feet of musk deer are adapted for climbing in rough terrain. Like the Chinese water deer, a cervid, they have no antlers, but the males do have enlarged upper canines, forming sabre-like tusks. The dental formula is similar to that of true deer: Failed to parse (Missing texvc executable; please see math/README to configure.): \tfrac{ 0.1.3.3}{ 3.1.3.3}

Deer (singular and plural) are the ruminant mammals forming the family Cervidae. Species in the Cervidae family include white-tailed deer, mule deer such as black-tailed deer, elk, moose, red deer, reindeer (caribou), fallow deer, roe deer and chital. Male deer of all species (except the Chinese water deer) and also female reindeer grow and shed new antlers each year. In this they differ from permanently horned animals such as antelope; these are in the same order as deer and may bear a superficial resemblance. The musk deer of Asia and water chevrotain (or mouse deer) of tropical African and Asian forests are not usually regarded as true deer and form their own families, Moschidae and Tragulidae, respectively.

The word "deer" was originally broad in meaning, but became more specific over time. In Middle English der (Old English dēor) meant a wild animal of any kind. This was as opposed to cattle, which then meant any sort of domestic livestock that was easy to collect and remove from the land, from the idea of personal-property ownership (rather than real estate property) and related to modern chattel (property) and capital. Cognates of Old English dēor in other dead Germanic languages have the general sense of "animal", such as Old High German tior, Old Norse djur or dȳr, Gothic dius, Old Saxon dier, and Old Frisian diar.