Flat, or flatness, describes an object or condition that is very smooth or level.In Great Britain, Ireland and Australia, as well as in English spoken in Commonwealth countries such as India, a "flat" means an apartment.
In mathematics, especially in set theory, a setA is a subset of a set B, or equivalently B is a superset of A, if A is "contained" inside B, that is, all elements of A are also elements of B. A and B may coincide. The relationship of one set being a subset of another is called inclusion or sometimes containment.
