Turtles are reptiles of the order Chelonii or Testudines characterised by a special bony or cartilaginous shell developed from their ribs and acting as a shield. Turtle may refer to the chelonian order as a whole (American English) or to fresh-water and sea-dwelling chelonians (British English).
The order Chelonii or Testudines includes both extant (living) and extinct species. The earliest known turtles date from , making turtles one of the oldest reptile groups and a more ancient group than lizards, snakes orcrocodiles. Of the many species alive today, some are highly endangered.
Like all other extant reptiles, turtles are ectotherms—their internal temperature varies according to the ambient environment, commonly called cold-blooded. However, because of their high metabolic rate, leatherback sea turtles have a body temperature that is noticeably higher than that of the surrounding water.
Turtles are classified as amniotes, along with reptiles, dinosaurs (including birds), and mammals. Like other amniotes, turtles breathe air and do not lay eggs underwater, although many species live in or around water.