Union

In set theory, union (denoted as ∪) is a set operation between two sets that forms a set containing the elements of both sets.

Formally, union A ∪ B means that if a ∈ A ∪ B, then a ∈ A ∨ a ∈ B, where ∨ - is logical or. We see this connection between ∪ and ∨ symbols.

Union operation is:
 * associative - (A ∪ B) ∪ C = A ∪ (B ∪ C)
 * commutative - A ∪ B = B ∪ A.