Difference between Apartment and Flat?

Generally there is no difference between an apartment and flat. Both words refer to a self-contained residential unit with its own front door, kitchen, toilet, and bathroom. Both are personal residence consisting of a series of rooms located on the same floor in a larger and singular building.

In some parts of the world, the word apartment refers to a purpose-built unit in a building, whereas the word flat means a converted unit in an older building, usually a big house.

A flat, similar to an apartment, is a housing unit that's self-contained but is part of a larger building with several units. In India, the word flat is used to refer to multi-storey dwellings that have lifts. While the words apartment and flat are often used interchangeably, some people refer to single-storied units as flats because of their "flat" nature.