An apartment 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).

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[citation needed].

Some apartment-dwellers own their own 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..