Closed sets are actually well named - a closed set is a set which is closed under the operation of taking limits. It's open sets which are badly named. Open se

Closed sets are actually well named - a closed set is a set which is closed under the operation of taking limits. It's open sets which are badly named. Open sets are used for distinguishing between points - i.e., two points (x,y) are topologically distinct if you can find two disjoint open sets x \in X, y \in Y with X \cap Y == \emptyset. Coming up with a collection of sets which are good for this purpose is totally NOT the opposite of being closed. "Distinguishing sets" might have been a better name.