The word
chelonian is popular among veterinarians, scientists, and conservationists working with these animals as a catch-all name for any member of the superorder Chelonia, which includes all turtles living and extinct, as well as their immediate ancestors.
The meaning of the word
turtle differs from region to region. In North America, all chelonians are commonly called turtles, including terrapins and tortoises. In Great Britain, the word
turtle is used for sea-dwelling species, but not for tortoises.
The term
tortoise usually refers to any land-dwelling, non-swimming chelonian.
https://en.wikipedia.org/wiki/Turtle
