Idempotency is a property of certain operations in mathematics and computer science where performing the operation multiple times has the same effect as performing it just once. For example, if you send a request to delete a file, doing it once or multiple times will result in the file being deleted, with no additional effect after the first deletion. In the context of web services, HTTP methods like PUT and DELETE are designed to be idempotent. This means that if a client sends the same request multiple times, the outcome remains unchanged, ensuring reliability and consistency in communication between systems.