A topological space is a set X, and a collection of subsets of X, C such that both the empty set and X are contained in C and the union of any subcollection of sets in C and the intersection of any finite subcollection of sets in C are also contained within C. The sets in C are called open sets. Their complements relative to X are called closed sets.

Given two topological spaces, X and Y, a map f from X to Y is continuous if for every open set U of Y, f^{−1}(U) is an open set of X.