Idempotent

A projection operator is idempotent.

An HTTP request using the PUT verb should be idempotent, so that it can be sent to the server any number of times with the same end result.

Every finite semigroup has an idempotent element.

Every group has a unique idempotent element: namely, its identity element.

Since the AND logical operator is commutative, associative, and idempotent, it distributes with respect to itself.