A set S and a binary operator * are said to exhibit closure if applying the binary operator to two elements S returns a value which is itself a member of S. The closure of a set A is the smallest closed set containing A. Closed sets are closed under arbitrary intersection, so it is also the intersection of all closed sets containing A. Typically, it is just A with all of its accumulation points. The term "closure" is also used to refer to a "closed" version of a given set. The closure of a set can be defined in several equivalent ways, including 1. The set plus its limit points, also called "boundary" points, the union of which is also called the "frontier." 2. The unique smallest closed set containing the given set.

binary operator | boundary set | closed set | connected set | existential closure | reflexive closure | sequence | tight closure | topology | transitive closure