Oxford Advanced Learner’s Dictionary, 8th edition
Flat:
noun. [ countable ] ( BrE ) a set of rooms for living in, including a kitchen, usually on one floor of a building.
Apartment：
noun. ( especially NAmE ) a set of rooms for living in, usually on one floor of a building
Does that means Flat is used in BrE and Apartment is used in AmE? And is there any other difference for the two words when one uses it?
I feel that flat is used more widely, is that right?
Best Answer
Flat is used in British English, and apartment is used in North American English. The exact meaning of the word apartment depends on where you live.
In large parts of Canada and in or near New York City, it is used for a residence in a multi-unit building; this meaning is the one given by OALD, and is a synonym of the British word flat.
In most of the rest of the U.S. and on the West Coast of Canada, the word apartment is reserved for a rented residence in a multi-unit building; if the residences in the building are individually owned, they are called condos.
As Ben Hocking says in his comment, an apartment doesn't have to be on just one story (although the majority of them are). But if you live in a multistory residence which doesn't have anything either above or below your unit, even though there are residences attached on both sides, this is more commonly called a townhouse. If there are just two residences in the same building, you have a duplex.
See also this question.