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).

A common alternative term for apartment is flat. The term apartment is favored in North America, whereas the term flat is commonly, but not exclusively, used in the United Kingdom, Ireland and the 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.

In the US, 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. In some locations, particularly the United States, the word denotes a rental unit owned by the building owner, and is not typically used for a condominium.

In the UK, some flat owners own a share in the company that owns the freehold of the building. This 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.

thumb|right|250px|Apartment Complex in Daegu, South Korea. In some countries the word unit is a more general term referring to both apartments and rental business suites. The word 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." In Australia a unit refers to flats, apartments or even semi detached houses.

When there is no tenant occupying an apartment, the apartment owner or landlord is said to have a vacancy. For apartment landlords, each vacancy represents a loss of income from rent-paying tenants for the time the apartment is vacant (i.e., unoccupied). Landlords' objectives are often to minimize the vacancy rate for their units. The owner of the apartment, typically when transferring possession to the occupant, gives him/her the key to the apartment entrance and any other keys needed, such as a common key to the building or any other common areas and a mailbox key. When the occupant(s) move out, these keys are typically returned to the owner.