What is the difference between a turtle , a terrapin and a tortoise? According to the all-knowing Wikipedia.org/english, long may it reign, it depends where you are speaking from.
British language refers to turtles as seagoing critters, terrapins as the same but from brackish waters, and tortoises as the landgoing versions.
American English is both more and less clear. Turtles = all freshwater and some landroaming critters that look turtle-like, sea turtles for those oceanic ones; tortoises is only for those members of the tortoise family, Testudinadae, while terrapin refers to only one species: the diamondback terrapin.
Our Aussie firends have cut to the chase and use only 2 terms: turtle for all forms of water-living turtloids, and tortoise for the land dwellers.
Apparently, if you want to be a cool daddy-o and impress your veterinary friends, the latest term, encompassin all the turtle/terrapin/tortoise brethren there are, were, or ever will be is
chelonian. Which makes it sound like you are keeping some Star Trek reject in your basement:
"Before I went to school today I made sure that my chelonian, Duuuuk Zsup, had plenty of yummy bugs to eat. Unfortunately, his home planet still has not called us back. "