flatflat3adverbwith no parts that are higher or lower than the rest: He lay flat on his back on the floor.flatflatflatterflattest1adjective (flatter, flattest)1smooth, with no parts that are higher or lower than the rest: The countryside in Holland is very flat.a flat surface2A tyre that is flat does not have enough air inside it.flatflatflats2noun (British) (Americanapartment)a group of rooms for living in, usually on one floor of a house or big buildingword buildingA tall building with a lot of flats in it is called a block of flats.