Lamb is a young sheep. The sheep (Ovis aries) is a quadrupedal, ruminant mammal typically kept as livestock. Like all ruminants, sheep are members of the order Artiodactyla, the even toed ungulates. Although the name "sheep" applies to many species in the genus Ovis, in everyday usage it almost always refers to Ovis aries. Numbering a little over one billion, domestic sheep are also the most numerous species of sheep. An adult female sheep is referred to as a ewe, an intact male as a ram or occasionally a tup, a castrated male as a wether, and a younger sheep as a lamb.