The largest living species of turtle (and fourth-largest reptile) is the leatherback turtle, which can reach over 2.7 m (8 ft 10 in) in length and weigh over 500 kg (1,100 lb). The term chelonian is used as a formal name for members of the group. Testudines is the official order name due to the principle of priority. The order has also been historically known as Chelonii ( Latreille 1800) and Chelonia (Ross and Macartney 1802), which are based on the Ancient Greek word χελώνη ( chelone) 'tortoise'. The name of the order, Testudines ( / t ɛ ˈ s tj uː d ɪ n iː z/ teh- STEW-din-eez), is based on the Latin word testudo 'tortoise' and was coined by German naturalist August Batsch in 1788. In Australia, which lacks true tortoises (family Testudinidae), non-marine turtles were traditionally called tortoises, but more recently turtle has been used for the entire group. In Britain, the name is used for sea turtles as opposed to freshwater terrapins and land-dwelling tortoises. In North America, it may denote the order as a whole. It is a common name and may be used without knowledge of taxonomic distinctions. The word turtle is borrowed from the French word tortue or tortre 'turtle, tortoise'. As a result of these pressures, many species are extinct or threatened with extinction. Turtle habitats around the world are being destroyed. Sea turtles are often killed accidentally as bycatch in fishing nets. Turtles have been hunted for their meat, for use in traditional medicine, and for their shells.

Some terrestrial and freshwater species are widely kept as pets. Turtles have appeared in myths and folktales around the world. Sea turtles are the only reptiles that migrate long distances to lay their eggs on a favored beach. Many turtles migrate short distances seasonally. They are generally opportunistic omnivores and mainly feed on plants and animals with limited movements. Turtles are ectotherms or 'cold-blooded', meaning that their internal temperature varies with their direct environment. The carapace bones develop from ribs that grow sideways and develop into broad flat plates that join up to cover the body. Its outer surface is covered in scales made of keratin, the material of hair, horns, and claws.

Turtle shells are made mostly of bone the upper part is the domed carapace, while the underside is the flatter plastron or belly-plate. Like other amniotes (reptiles, birds, and mammals) they breathe air and do not lay eggs underwater, although many species live in or around water. They are found on most continents, some islands and, in the case of sea turtles, much of the ocean. There are 360 living and recently extinct species of turtles, including land-dwelling tortoises and freshwater terrapins. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked turtles), which differ in the way the head retracts. Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs.

Tortoise skeleton sketch side