An apartment (in American and Canadian English) or a flat (in British English) is a self-contained housing unit (a type of residential real estate) that occupies only part of a building, correctly, on a single level without a stair. Such a building may be called an apartment building, apartment complex (in American English), 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. In Scotland it is called a block of flats or, if it's a traditional sandstone building, a tenement, which has a pejorative connotation elsewhere. Apartments may be owned by an owner/occupier, by leasehold tenure or rented by tenants (two types of housing tenure).
The term apartment is favoured 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) and also is the preferred term in Ireland. In the UK, the term apartment is more usual in professional real estate and architectural circles where otherwise the term flat is commonly, but not exclusively, for an apartment without a stair (hence a 'flat' apartment). Technically multi-storey apartments are referred to as 'duplex' (or 'triplex') indicating the number of floors within the property. Usage generally follows the British in Singapore, Hong Kong and most Commonwealth nations.