A union-closed set is a nonempty finite collection of distinct nonempty finite sets which is closed under union.
Union-Closed Set
See also
Union-Closed Sets ConjectureExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Union-Closed Set." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Union-ClosedSet.html