In computing, an idempotent operation is one that has no additional effect if it is called more than once with the same input parameters. For example, removing an item by ID from a set can be considered an idempotent operation on the set.
In mathematics, an idempotent operation is one where f(f(x)) = f(x). For example, the abs() function is idempotent because abs(abs(x)) = abs(x) for all x.