The tail is the elongated section at the rear end of a bilaterian animal's body; in general, the term refers to a distinct, flexible appendage extending backwards from the midline of the torso. In vertebrate animals that evolved to lose their tails (e.g. frogs and hominid primates), the coccyx is the homologous vestigial of the tail. While tails are primarily considered a feature of vertebrates, some invertebrates such as scorpions and springtails, as well as snails and slugs, have tail-like appendages that are also referred to as tails.

Tail-shaped objects are sometimes referred to as "caudate" (e.g. caudate lobe, caudate nucleus), and the body part associated with or proximal to the tail are given the adjective "caudal" (which is considered a more precise anatomical terminology).

Animal tails are used in a variety of ways. They provide a source of thrust for aquatic locomotion for fish, cetaceans and crocodilians and other forms of marine life. Terrestrial species of vertebrates that do not need to swim, e.g. cats and kangaroos, instead use their tails for balance; and some, such as monkeys and opossums, have grasping prehensile tails, which are adapted for arboreal locomotion.

Many animals use their tail for utility purposes, for example many grazing animals, such as horses and oxen, use their tails to drive away parasitic flies and sweep off other biting insects. Some animals with broad, furry tails (e.g. foxes) often wrap the tail around the body as means of thermal insulation like a blanket.

Some species' tails serve aggressive functions, either predatorily or defensively. For example, the tails of scorpions have a stinger that contain venom, which can be used to either kill large prey or to fight off a threat. Similarly, stingrays have a thickened spine that can deliver penetrating trauma. Thresher sharks are known to use their long tails to stun prey. Many species of snakes wiggle their tails as a lure to attract prey, who may mistake the tail as a worm. The extinct armored dinosaurs (stegosaurs and ankylosaurs) have tails with spikes or clubs as defensive weapons against predators.

Tails are also used for communication and signalling. Most canines use their tails to communicate mood and intention. Some deer species flash the white underside of their tails to warn other nearby deer of possible danger, beavers slap the water with their tails to indicate danger, felids raise and quiver their tails while scent-marking, and canids (including domestic dogs) indicate emotions through the positioning and movement of their tails. Rattlesnakes perform tail vibration to generate a distinct rattling noise that signals aggression and warns potential predators to stay away.

Some species of lizard (e.g. geckos) can self-amputate ("cast") their tails from their bodies to help them escape predators, which are either distracted by the wriggling detached tail or only manages to seize the severed 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 and contains only cartilage, not bone. Various species of rat demonstrate a similar function with their tails, known as degloving, in which the outer layer is shed in order for the animal to escape from a predator.

Most birds' tails end in long feathers called rectrices. These feathers are used as a rudder, helping the bird steer and maneuver in flight; they also help the bird to balance while it is perched. In some 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 other species, including woodpeckers and woodcreepers, allow them to brace themselves firmly against tree trunks.