An apartment (or flat in Britain and most other Commonwealth countries) is a self-contained housing unit that occupies only part of a building. Apartments may be owned (by an owner-occupier) or rented (by tenants).
Some apartment-dwellers own their apartments, 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 apartments 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 connotes a residential unit or section in a building. Apartment 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 apartments. When there is no tenant occupying an apartment, the lessor is said to have a vacancy. For apartment lessors, each vacancy represents a loss of income from rent-paying tenants for the time the apartment is vacant (i. e., unoccupied). Lessors' objectives are often to minimize the vacancy rate for their units. The owner of the apartment typically transfers possession to the occupant(s) by giving him/her the key to the apartment entrance door(s) and any other keys need 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 should typically be returned to the owner.