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, flat complex, 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 in some cities flat is used for a unit which is part of a house containing two or three units, typically one to a floor). 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 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. This usage has also been appearing in British English where apartment is used by developers to denote expensive ‘flats’ in exclusive and expensive residential areas in, for example, parts of London such as Belgravia and Hampstead.