My son wants to know the difference between a turtle and a tortoise.
According to Merriam-Webster Turtle is a broader term. Totroises are a subsets of turtles:
So, while in general the terms are used interchangeably, tortoise is more specific.
Turtles swim.
Tortoises don’t.
Terrapins can if they want to, but don’t always.
You know box turtles? They ain’t turtles at all. Nope. They’re terrapins.
-Rue.
It’s quite simple, Sterling.
“Turtle” is used for all the reptiles with a “shell” (if you notice, he has skin joining two pieces, the upper one being called the “carapace” and the bottom one the “plastron”). Taxonomically, they’re Order Chelonia. It’s also used for the majority of those critters which live mostly in the water.
A tortoise is a turtle (broad sense) which lives almost totally on land, rarely if ever entering the water. I think they represent one family in the turtle order, but I’m not certain and don’t have references on hand to check it.
The Order Chelonia (which contains about 12 Families) contains all turtles, tortoises and terrapins. Tortoises belong to the Family Testudinae, while turtles occupy all the other 11 families. “Terrapin” is something of a generic term applied to large freshwater turtles.
Kee-rect. But most workers seem to prefer the ordinal name Testudines over Chelonia, these days 
. Though I see people using both in weird fashions - i.e. quotes such as “…Chelonians ( Order Testudines )…” . I believe Chelonia is the older name and Testudines was once a family name, that has now been upgraded.
I think you were correct at one time, in fact I think Testudines may have been the name for the “tortoise family”. But apparently this has been tossed out. To quote from the fairly definitive Turtles of the World by Carl H. Ernst and Roger W. Barbour ( 1989, Smithsonian Instituition ):
“All living shelled reptiles are turtles, but the terms tortoise and terrapin have also been applied, and these have different meanings in various parts of the world. Tortoise is best applied to terrestrial turtles. Terrapin is usually applied to edible, more or less aquatic, hard-shelled turtles.”
- Tamerlane
Okay, you made me go check a little more closely . The tortoise family is indeed still Testudinidae. Mea Culpa. But Chelonia does seem to have fallen out of favor as the ordinal name.
Well I was half-right, which is better than entirely wrong 
.
Ack. That’s what I get for being hard-of-seeing. Didn’t catch that extra “id”
Let’s try this again, using my preferred taxonomical method: cladistics!
Turtles (again, including tortoises) are known as Testudines, as you mentioned, with tortoises occupying the clade known as Testudinidae ( ) (which are most closely related to the various pond turtles). One of the characteristics of tortoises which sets them apart from the rest of turtledom is the fact that they are, of course, exclusively terrestrial. Essentially, all tortoises are turtles, but not all turtles are tortoises (say that ten times fast!).
As Tamerlane alluded, “Chelonia” is largely only used in Linnaean circles.
Not sure where this comes into the whole picture, but tortoises are, AFAIK, exclusively vegetarian, and turtles/terrapins are carniverous to omniverous.
“Turtle” is a corruption of the word “tortoise,” perpetrated by sailors. Since sailors generally saw “toroises” in the water, the distinction came to be made that “turtles” were seagoing and “tortoises” were terrestrial.
BTW, the Bible makes references to turtles as in “the sound of the turtle” or “voice of the turtle,” which seems a little odd, but that is actually referring to doves, as in “turtle doves,” which has no tie-in with the reptilian types of turtles. “Turtle dove” is similar to “moo cow” or “hoot owl.” That is, an adjective describing the sound it makes, which to some resembles “Tur-tur.”
Source–Isaac Asimov.
Aw, thank you Polycarp.
Sterling enjoyed seeing hisself referred to by name here.