Definition:Idempotence/Operation

Definition
Let $\circ: S \times S \to S$ be a binary operation.

If all the elements of $S$ are idempotent, then the term can be applied to the operation itself:

The binary operation $\circ$ is idempotent iff:
 * $\forall x \in S: x \circ x = x$

Examples of idempotent operations are set union $\cup$ and set intersection $\cap$.