What is a Chelonian?

Testudines includes the Chelonia and some of their extinct relatives from the extentive turtle fossil record. 

Chelonia (ke-lō′ni-a) is a group name given to extant animals commonly known as turtles, tortoises and terrapins. Alternative scientific names for the group include Chelonii, Testudinata, Testudines.

Chelonia is the Greek word for tortoise whereas testudo (meaning "moveable shelter") is Latin.

According to where you live the common names for the group may vary. In North America, all chelonians are commonly called turtles, whereas in the UK, terrestrial turtles are called tortoises. Terrapins, in the UK, refers to a small group of freshwater turtles called sliders in the US. In Spanish, all turtles are called tortuga, and are categorised as tortuga marina, (marine) tortuga de rio (freshwater) and tortuga terrestre (land).

There are approximately 300 extant species grouped into 14 families.