noun
closed set
In topology, a set whose complement is an open set; equivalently, a set containing all its limit points.
実数直線において、閉区間 [0,1] は閉集合である。
In the real line, the closed interval [0,1] is a closed set.
開集合 (open set) is the complement concept: a set where every point has a neighborhood contained in the set. 閉集合 is its dual.
Compound of 閉 (へい, 'closed') + 集合 (しゅうごう, 'set'), a direct translation of the English mathematical term 'closed set'.