An apartment (in American English) or flat (in British English) is a self-contained housing unit (a type of residential real estate) that occupies only part of a building. Such a building may be called an apartment building, apartment house (in American English), block of flats, tower block, high-rise or, occasionally mansion block (in British English), especially if it consists of many apartments for rent. Apartments may be owned by an owner/occupier by leasehold tenure or rented by tenants (two types of housing tenure).

The term apartment is favored in North America (although flat is used in the case of a unit which is part of a house containing two or three units, typically one to a floor), whereas the term flat is commonly, but not exclusively, used in the United Kingdom, Ireland, Singapore, Hong Kong and most Commonwealth nations.

In Malaysian English, flat often denotes a housing block of lesser quality meant for lower-income groups, while apartment is more generic and may also include luxury condominiums.