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Terminology

Differences exist in usage of the common terms turtle, tortoise, and terrapin, depending on the variety of English being used; usage is inconsistent and contradictory. These terms are common names and do not reflect precise biological or taxonomic distinctions.

The American Society of Ichthyologists and Herpetologists uses "turtle" to describe all species of the order Testudines, regardless of whether they are land-dwelling or sea-dwelling, and uses "tortoise" as a more specific term for slow-moving terrestrial species. General American usage agrees; turtle is often a general term; tortoise is used only in reference to terrestrial turtles or, more narrowly, only those members of Testudinidae, the family of modern land tortoises; and terrapin may refer to turtles that are small and live in fresh and brackish water, in particular the diamondback terrapin (Malaclemys terrapin). In America, for example, the members of the genus Terrapene dwell on land, yet are referred to as box turtles rather than tortoises.

British usage, by contrast, tends not to use "turtle" as a generic term for all members of the order, and also applies the term "tortoises" broadly to all land-dwelling members of the order Testudines, regardless of whether they are actually members of the family Testudinidae. In Britain, terrapin is used to refer to a larger group of semiaquatic turtles than the restricted meaning in America.

Australian usage is different from both American and British usage. Land tortoises are not native to Australia, and traditionally freshwater turtles have been called "tortoises" in Australia. Some Australian experts disapprove of this usage—believing that the term tortoises is "better confined to purely terrestrial animals with very different habits and needs, none of which are found in this country"—and promote the use of the term "freshwater turtle" to describe Australia's primarily aquatic members of the order Testudines because it avoids misleading use of the word "tortoise" and also is a useful distinction from marine turtles.