Tortoise is a reptile of tortoise, which is characterized by a special bone or cartilage like shell developed from its ribs and used as a shield. "Turtle" can refer to the whole command (in American English), or freshwater and dolphin testicles (in British English).

The order of subjects included living and extinct species. The earliest members of the group date back 157 million years, making tortoises one of the oldest reptile groups, older than snakes or crocodiles. Some of the 327 known species that survive today are endangered.

Tortoises are exogenous animals (usually called cold-blooded animals), which means that their internal temperature will change according to the surrounding environment. However, due to the high metabolic rate of leatherback turtles, their body temperature is significantly higher than the surrounding water temperature.

Tortoises, along with other reptiles, birds and mammals, are listed as amniotic. Like other amniotic animals, turtles breathe air and do not spawn underwater, although many species live in or around the water.