Apartment refers to a self-contained housing unit that occupies only part of a building, generally on a single level.
In different regions, apartment may have different meanings:
1. In American English, it typically means a rented residential unit in a multi-unit building
2. In British English, it's more commonly called a "flat"
3. Apartment can also refer to luxury residential units in some contexts
4. The term is sometimes used interchangeably with "condominium"