Union

Set

 context $X,Y$ … set definiendum $x\in X \cup Y$ postulate $x\in X\lor x\in Y$

Discussion

The 2-ary set construction $X \cup Y$ is commutative and idempotent.

Notice that $X \cup \emptyset = X$.

The union and intersection are associative and distributive with respect to another.

Wikipedia: Union