Turtle, Tortoise, or Terrapin
The use of the terms, turtle, tortoise, and terrapin is not straightforward. All animals with a shell and backbone are classified as turtles, and the word turtle is used to describe all members. In general use, the term tortoise usually refers to any land-dwelling turtle, although in Great Britain, tortoise refers to any turtle other than a sea turtle, and scientifically tortoises are the Testudinidae family, only one of the 14 extant turtle families. Terrapin is a Algonquian word for turtle and usually refers to small, edible, hard-shell turtles found in aquatic or brackish waters.
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. Chelonia is based on the Greek word kelone, for armour or interlocking shields
Some languages do not have this problem, as all of these are referred to by the same name. For example, in Spanish, the word tortuga is used for turtles, tortoises, and terrapins, though the type they belong to is usually specified and added to the name, as marina for sea turtles, del río for freshwater species, and terrestre for tortoises.
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