Full Idea
Axiom of Unions: For every collection of sets there exists a set that contains all the elements that belong to at least one of the sets in the collection.
Gist of Idea
Unions: There is a set of all the elements which belong to at least one set in a collection
Source
Brian Clegg (Infinity: Quest to Think the Unthinkable [2003], Ch.15)
Book Reference
Clegg,Brian: 'Infinity' [Robinson 2003], p.206