An apartment or flat is a self-contained housing unit that occupies only part of a building. Apartments or flats may be owned (by an owner-occupier) or rented (by tenants).

The term “apartment” is favored in North America, whereas the term “flat” is sometimes, but not exclusively used in the United Kingdom and most other English-speaking areas and Commonwealth nations.

Some apartment-dwellers own their flats, either as co-ops, in which the residents own shares of a corporation that owns the building or development; or in condominiums, whose residents own their flats and share ownership of the public spaces. Most apartments are in buildings designed for the purpose, but large older houses are sometimes divided into apartments. The word apartment or flat connotes a residential unit or section in a building. apartment or flat building owners, lessors, or managers often use the more general word units to refer to apartments. Units can be used to refer to rental business suites as well as residential flats. When there is no tenant occupying an apartment, the lessor is said to have a vacancy.

For apartment or flat lessors, each vacancy represents a loss of income from rent-paying tenants for the time the apartment or flat is vacant (i.e., unoccupied). Lessors’ objectives are often to minimize the vacancy rate for their units. The owner of the apartment or flat typically when transferring possession to the occupant(s) gives him/her the key to the apartment or flat entrance door(s) and any other keys needed to live there, such as a common key to the building or any other common areas, and an individual unit mailbox key. When the occupant(s) move out, these keys are typically returned to the owner.