An apartment (American English), flat (British English) or unit (Australian English) is a self-contained housing unit (a type of residential real estate) that occupies only part of a building, generally on a single storey. There are many names for these overall buildings, see below.
It may be only residential, in which case it might also be called a tower block, or it might include other functions such as hotel, offices, or shops. There is no clear difference between a tower block and a skyscraper, although a building with fifty or more stories is generally considered a skyscraper. High-rise buildings became possible with the invention of the elevator (lift) and cheaper, more abundant building materials. Their structural system usually is made of reinforced concrete and steel.
A low-rise building and mid-rise buildings have fewer storeys, but the limits are not always clear. Emporis defines a low-rise as an enclosed structure below 35 metres [115 feet] which is divided into regular floor levels. The city of Toronto defines a mid-rise as a building between 4 and 12 stories.
Most apartments are in buildings designed for the purpose, but large older houses are sometimes divided into apartments. The word apartment denotes a residential unit or section in a building. In some locations, particularly the United States, the word connotes a rental unit owned by the building owner, and is not typically used for a condominium. In the England and Wales, some flat owners own shares in the company that owns the freehold of the building as well as holding the flat under a lease.
This arrangement is commonly known as a share of freehold flat. The freehold company has the right to collect annual ground rents from each of the flat owners in the building. The freeholder can also develop or sell the building, subject to the usual planning and restrictions that might apply. This situation does not happen in Scotland, where long leasehold of residential property was formerly unusual, and is now impossible.
In other places the terms are interchangeable. The term apartment is favored 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 used commonly, but not exclusively, for an apartment on a single level (hence a 'flat' apartment). In some countries the word unit is a more general term referring to both apartments and rental business suites.
The word 'unit' is generally used only in the context of a specific building; e.g., This building has three units or I'm going to rent a unit in this building, but not I'm going to rent a unit somewhere. Some buildings can be characterized as 'mixed use buildings', meaning part of the building is for commercial, business, or office use, usually on the first floor or first couple of floors, and one or more apartments are found in the rest of the building, usually on the upper floors.