The donkey or ass, Equus asinus, is a member of the Equidae or horse family, and an odd-toed ungulate. The words donkey and ass are applied to the domesticated E. asinus. The animal considered to be its wild ancestor is the African Wild Ass, also E. asinus.

Colloquially, the term "ass" is often used today to refer to a larger, horse-sized animal, and "donkey" to a smaller, pony-sized one. In the western United States, a small donkey is sometimes called a burro (from the Spanish word for the animal).

A male donkey or ass is called a jack, a female a jenny, and offspring less than one year old, a foal (male: colt, female filly).

While different species of the Equidae family can interbreed, offspring are almost always sterile. Nonetheless, horse/donkey hybrids are popular for their durability and vigor. A mule is the offspring of a jack (male donkey) and a mare (female horse). The much rarer successful mating of a male horse and a female donkey produces a hinny.

Asses were first domesticated around 3000 BCE Rossel S, Marshall F et al. "Domestication of the donkey: Timing, processes, and indicators." PNAS 105(10):3715-3720. March 11, 2008. Abstract, approximately the same time as the horse, and have spread around the world. They continue to fill important roles in many places today and domesticated species are increasing in numbers (although the African wild ass and another relative, the Onager, are endangered species). As "beasts of burden" and companions, asses and donkeys have worked together with humans for centuries.

:For other uses, see Mule (disambiguation)

In its common modern meaning, a mule is the offspring of a male donkey and a female horse, which is classified as a kind of F1 hybrid. The much rarer offspring of a male horse and a female donkey is called a hinny. The term "mule" (Latin mulus) was formerly applied to the infertile offspring of any two creatures of different species. The mule, easier to breed and usually larger in size than a hinny, has monopolized the attention of breeders . The chromosome match-up more often occurs when a donkey is the sire and the horse is the dam. Sometimes people let a stallion (male horse) run with a jenny (female donkey) for as long as six years before she becomes pregnant. Mules and hinnies are almost always sterile (see fertile mules below for rare cases). The sterility is attributed to the differing number of chromosomes of the two species: donkeys have 62 chromosomes, whereas horses have 64.

A female mule that has estrus cycles and can carry a fetus is called a "molly" and can occasionally occur naturally as well as through embryo transfer.