It's an important term in computer science. There is a good explanation on StackOverflow which I found by searching for "define: idempotent":

https://stackoverflow.com/a/1077421/111327

> 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 from a set can be considered an idempotent operation on the set.