The union of two sets is the set of things that are members of either. Formally, x ∈ S ? R iff x ∈ S ∨ x ∈ R. However, the union of a single set, ?A, is the union of all its subsets, that is x ∈ ?A iff (∃S)(S ∈ A & x ∈ S). This connects with the first notion via the fact that ?S,R = S ? R.
Philosophy dictionary. Academic. 2011.