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A terrapin is a small turtle that lives in fresh or slightly salty water — ponds, rivers, marshes — rather than in the sea. It has a hard shell, swims well, and comes out onto rocks or logs to sit in the sun. The word matters mostly for the kind of place the animal lives: a sea turtle is a 'turtle', a land one is a 'tortoise', and the small water ones in between are terrapins. That is the tidy answer. In real life people are looser than that. In American English, 'turtle' covers almost everything, and 'terrapin' is used mainly for particular species (the diamondback terrapin is famous, and it is a university's team name in Maryland), or in cooking, because people used to eat them in soup. In British English 'terrapin' is the ordinary word for the small pet-shop water turtle a child keeps in a tank, so a British speaker will say terrapin where an American says turtle. The word is not common in daily conversation anywhere; you meet it in books about animals, in pet shops, and in place or team names. It is a plain, neutral word — not formal, not , just not used often.
澤龜、水龜（淡水的小烏龜）。台灣日常多半就講「烏龜」或「巴西龜」（寵物店最常見的那種），terrapin 這個細分的詞中文沒有很通行；不要用「海龜」（那是 sea turtle）或「陸龜」（tortoise）。
組成Not built from English pieces. It comes from a Native American word — Powhatan (a language of Virginia) *tōrəp, meaning a turtle — borrowed into English as 'torup' and then reshaped with an English ending. The Latin word 'terra' (earth, land) may have pushed the spelling toward 'terr-', which is a little joke of history, since the terrapin is the water one.
原始詞典資料4 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
terrapinnoun/ˈtɛɹəpɪn/
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Any of several small turtles of the families Emydidae and Geoemydidae found throughout the world.
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obsoleteAny turtle.
- The third ſervice was made up of a loin of freſh pork with apple-ſauce, a kid ſmothered with onions, and a terrapin baked in the ſhell; […]
- The land-turtle, or terrapin, is much better known at Nice, as being a native of this county; yet the beſt are brought from the iſland of Sardinia. The ſoup or bouillon of this animal is always preſcribed here as a great reſtorative to conſumptive patients.
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obsoleteAny of several small turtles native to North America that live in brackish or fresh water, especially the diamondback terrapin (Malaclemys terrapin).
- As to the terrapin, I have not so much to say. The terrapin is a small turtle, found on the shores of Maryland and Virginia, out of which a very rich soup is made. It is cooked with wines and spices, and is served in the shape of a hash, with heaps of little bones mixed through it. […] I must, however, confess that the terrapin for me had no surpassing charms.
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obsoleteThe flesh of such a turtle used as food.
- The man who did not eat twice of terrapin would be held in small repute, as the Londoner is held who at a city banquet does not partake of both thick and thin turtle.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。