It all depends on how technical you want to be.
(I am writing as the spouse of a nationally recognized expert on wild turtles, tortoises and terrapins in the US.)
In the US there are 50 species of "chelonians" excluding sea turtles.
Among experts they are identified by strict Latin taxonimic names when precise species identification is required.
There is also a looser English taxonomy, but there is enough regional variation that experts fall back on Latin when confusion arises.
But, even experts in "casual" conversation use the word "turtle" to include "turtles, tortoises and terrapins"; e.g. "I study turtles."
Some non-experts refer to the shelled animals that live primarily in water (lakes, rivers and streams) as "turtles" and those that live primarily on land as "tortoises".
However, in the US northeast, the most common land dweller is the "box turtle ".
Bottom line, in casual conversations in the US the word "turtle" encompasses all those shelled creatures.
In discussion with passionate "turtle people" try to use the correct taxonomic name.
english.stackexchange.com/users/2944/John Satta
Your colleague is correct to an extent.
Generally in British English, tortoise refers to the land animal, turtle to the marine animal, and terrapin to the freshwater animal.
In U.S. English, turtle is used as a catch-all term for all animals of that type, but distinctive tortoises like the giant Galapagos tortoise will be called such.
english.stackexchange.com/users/None/
Yes.
American English often uses "turtle" for tortoise, in British English the two terms are not interchangeable, and (for what it's worth)
Indian English often uses tortoise as the generic term for both turtles and tortoises.
( The Hare and the Tortoise is never * The Hare and the Turtle , and Indian languages—and Japanese is similar according to a comment above—usually have the same words for both, and this is mapped to tortoise , not turtle .)
So it varies from region to region.
As evidence, the Compact Oxford English Dictionary (UK) says, under tortoise that it's "Called turtle in North America", and the American Heritage Dictionary of the English Language gives, under tortoise , definition 1b saying " Chiefly British A terrestrial or freshwater chelonian."
That they are distinct in British English can be seen from this exchange from Alice's Adventures in Wonderland :
“When we were little,” the Mock Turtle went on at last, more calmly, though still sobbing a little now
and then, “we went to school in the sea.
The master was an old Turtle—we used to call him Tortoise–—”
“Why did you call him Tortoise, if he wasn’t one?”
Alice asked.
“We called him Tortoise because he taught us,” said the Mock Turtle angrily.
“Really you are very dull!”
english.stackexchange.com/users/77/ShreevatsaR
In the NOAD is reported that turtle means: a slow-moving reptile, enclosed in a scaly or leathery domed shell into which it can retract its head and thick legs.
•
Family Testudinidae : numerous genera and species, including the European tortoise ( Testudo graeca ). (also sea turtle) a large marine reptile with a bony or leathery shell and flippers, coming ashore annually on sandy beaches to lay eggs.
•
Families Cheloniidae (seven species) and Dermochelyidae (the leatherback).
a freshwater reptile related to the turtles, typically having a flattened shell.
Called terrapin in South Africa and India and tortoise in Australia.
• Order Chelonia : several families, in particular Emydidae and Kinosternidae .
•
any reptile of this order, including the terrapins and tortoises.
It's then true that in American English the term turtle can be used to refer to an animal that in other English languages is called tortoise .
This is not something that happens only in American English, though.
Also other languages have a word equivalent to turtle (In Italian tartaruga ) and one equivalent to tortoise (in Italian testuggine ), but the first can be used instead of the second (even if it is less accurate).
To notice also that, for example, in Italian the European tortoise is called tartaruga .
english.stackexchange.com/users/252/kiamlaluno
Yes.
American speakers do use the term turtle to refer to both the land-based and marine reptiles .
Whereas many (not all)
BrEng speakers will differentiate between the two species.
I always thought it strange that the cartoon series my son used to watch, Teenage Mutant Ninja Turtles , back in the late 1980s was titled so.
It was obvious that the four pizza-obsessed ninja turtles; Raphael, Leonardo, Donatello and Michelangelo performed their heroic deeds on land and and hardly ever in sea or freshwaters.
Although I do remember they spent some time in the sewers....
And now nearly thirty years later, I find that Americans still prefer to use the term turtle .
In The Big Bang Theory, season 8 episode 17, Amy and Sheldon make an important announcement: Amy : We have some exciting news.
Leonard :
OK Sheldon :
As you know, Amy and I have been together a long time.
And a lot of things that I never thought were possible; now seem possible.
Penny : Okaaay...
Amy :
After a careful evaluation of our relationship, we decided that the time was right to take a step forward.
Leonard : ...
OK Sheldon :
Do you want to say it?
Amy :
Let's say it together.
Sheldon and Amy : We're getting a turtle!
We then see the couple in a pet shop choosing a tortoise .
This is all the more remarkable considering that Sheldon and Amy are scientists, Sheldon a theoretical physicist while Amy a neurobiologist.
english.stackexchange.com/users/44619/Mari-Lou A
Obviously there are two different animals — a tortoise and a turtle. But I have been told by a colleague that in the US the word  turtle  is used to describe both.  I find this odd as for example the Galapagos tortoise was fairly well known (old & big) and it would surely be a mistake to refer to it as a turtle.
Do Americans use the world 'turtle' as a generic word to mean 'tortoise'?
english.stackexchange.com
<american-english><nouns><british-english>
english.stackexchange.com/questions/10265
english.stackexchange.com/users/4221/lukehmu
The answer will depend on how technical you want to be and what region you're in. In casual conversation, the word "turtle" is used to include "turtles, tortoises and terrapins". Generally in British English, "tortoise" refers to the land animal while "turtle" refers to the marine animal, whereas in the US "turtle" is used as a catch-all tern for all animals of that type. On the other hand, in Indian English, "tortoise" is the generic term for both turtles and tortoises. In the US the species is called "chelonians" and this excludes sea turtles.There are lots of regional variations to identify the animal but the correct one is the Latin taxonomic form."Turtles " is  used to denote sea creatures and "tortoises" land animals ,by non-experts, especially in the UK. However, in the US, the preference is for the word "turtles"  as stated in the OED, except for specific species which carry the name "tortoise" such as the Giant Galapagos tortoise. In general, in the US the term "turtle" is also used for "tortoise" whereas in the UK, the terms are not interchangeable. Indian English often uses "tortoise"  as the generic term for both turtles and tortoises. The advice to avoid confusion is to use the correct taxonomic name.
The answer will depend on how technical you want to be and what region you're in. In casual conversation, the word "turtle" is used to include "turtles, tortoises and terrapins". Generally in British English, "tortoise" refers to the land animal while "turtle" refers to the marine animal, whereas in the US "turtle" is used as a catch-all tern for all animals of that type. On the other hand, in Indian English, "tortoise" is the generic term for both turtles and tortoises. British English speakers often distinguish between shelled animals living in the sea (turtles), on land (tortoises) and in freshwater (terrapins). In contrast, US English speakers often use the word 'turtle' for all three. The distinction is not completely accurate, so experts will often use the correct taxonomic names rather than these broad classes.
In U.S. English,  turtle encompasses all shelled creatures, both marine and land reptiles. Distinctive tortoises like the giant Galapagos tortoise will be called such. It asll depends on how techical you want to be.  The answer to this question will depend on how technical you want to be, but in general, in American English, the term "turtle" is used to refer to both turtles and tortoises.
In the US there are 50 species of "chelonians" excluding sea turtles. Lots of regional variations, but correct species identified by Latin name. Turtles used to denote sea creatures and "tortoises" land animals by non-experts, esp. In UK Prevalence of "turtle" in US English. Indian English use.
British English speakers often distinguish between shelled animals living in the sea (turtles), on land (tortoises) and in freshwater (terrapins). In contrast, US English speakers often use the word 'turtle' for all three.  The distinction is not completely accurate, so experts will often use the correct taxonomic names rather than these broad classes.
The answer to this question will depend on how technical you want to be. Generally, it is accepted that in American English, the term "turtle" is used to refer to both turtles and tortoises.