Carpet area plus the area under the flat's walls, and often its balconies. It has no legal definition, so builders count it differently.
Guide
Three numbers describe the same flat and only one of them is floor you can stand on. How they differ, and which to ask for first.
Resale flats are often quoted on it. Ask what it includes: a balcony counted in one quote and not in another makes two flats look the same size when they are not.