Flat, as per British English, means a set of rooms forming an individual residence, typically on one floor and within a larger building. Together they are also called a block of flats. The term apartment is preferred by Americans since it often gives an impression of luxury.
A flat is a set of rooms for living in, usually on one floor and part of a larger building. A 1BHK, is 1 Bedroom, Hall and Kitchen. A 2 bhk apartment, is 2 Bedrooms, Hall and Kitchen. A 3BHK, is 3 Bedrooms, Hall and Kitchen.