noun
open set
A set in a topological space where every point has a neighborhood contained in the set. Used in mathematics, especially topology.
実数直線において、区間 (0, 1) は開集合である。
In the real line, the interval (0, 1) is an open set.
閉集合 (closed set) is the complement of an open set; it contains all its limit points.
Compound of 開 (kai, 'open') + 集合 (shūgō, 'set'), a direct translation of the English mathematical term 'open set'.