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.

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.