The term means level and in one plane, which nowadays relates to many different things, such as a deflated tire, lying prone, and a dwelling on one story. Hence, the term flat is used to describe a one-level Flat.
there is no difference between an apartment and a flat. Both terms refer to a personal residence consisting of a series of rooms located on the same floor, within a larger, singular building.
A flat is a set of rooms for living in, usually on one floor and part of a larger building. A flat usually includes a kitchen and bathroom.