(abbrev. int), n. 1. the set of points lying strictly between the end points of an interval. 2. the set of all interior points of a given set in a topological space; the largest open subset of the set, equal to the union of all the open sets contained in the given set. The interior of A is denoted IntA or A°. Compare closure. 3. (of a simple closed curve) the bounded region determined by the curve, as ensured by the Jordan curve theorem.