The tail is the section at the rear end of an animal's body; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals, reptiles, and birds. While tails are primarily a feature of vertebrates, a few invertebrates including scorpions and springtails, as well as snails and slugs, have tail-like appendages that are at times referred to as tails. Tailed objects are at times referred to as "caudate" and the part of the body associated with or proximal to the tail are given the adjective "caudal".
Animal tails are used in a variety of ways. They provide a source of locomotion for fish and a few additional forms of marine life. Many land animals use their tails to brush away flies and additional biting insects. Some species, including cats and kangaroos, use their tails for balance; and some, such as New World monkeys and opossums, have what're known as prehensile tails, which are adapted to allow them to grasp tree branches.
Tails are additionally used for social signaling. Some deer species flash the white underside of their tails to warn additional nearby deer of possible danger, beavers slap the water with their tails to indicate danger, and canids (including domestic dogs) indicate emotions through the positioning and movement of their tails. Some species' tails are armored, and some, such as those of scorpions, contain venom.
Some species of lizard can detach ("cast") their tails from their bodies. This can help them to escape predators, which are either distracted by the wriggling, detached tail or left with only the tail while the lizard flees. Tails cast in this manner generally grow back over time, though the replacement is typically darker in colour than the original.
Most birds' tails end in long feathers called rectrices. These feathers are used as a rudder, helping the bird steer and manoeuvre in flight; they additionally help the bird to balance while it is perched. In a few species—such as birds of paradise, lyrebirds, and most notably peafowl—modified tail feathers play an important role in courtship displays. The extra-stiff tail feathers of additional species, including woodpeckers and woodcreepers, allow them to brace themselves firmly against tree trunks.
The tails of grazing animals, such as horses, are used both to sweep away insects and positioned or moved in ways that indicate the animal's physical or emotional state.
Human embryos have a tail that measures about one-sixth of the size of the embryo itself. As the embryo develops into a fetus, the tail is absorbed by the growing body. Infrequently, a child is born with a ’"soft tail", which contains no vertebrae, but only blood vessels, muscles, and nerves, but this is regarded as an abnormality rather than a vestigial true tail, even when such an appendage is located where the tail would be expected.