idempotent

The term idempotent is used to describe an operation that will produce the same results if executed once or multiple times. It means that the modified state remains the same after the first call. The term was introduced by Benjamin Peirce (1870) in the context of elements of algebras that remain invariant when raised to a positive integer power (e.g., a² = a), and literally means "(the quality of having) the same power", from idem + potence (same + power).

The HTTP methods PUT and DELETE in a RESTful API are idempotent methods. The GET method is a safe method (or nullipotent), meaning that calling it produces no side-effects.

Sources: http://en.wikipedia.org/wiki/Idempotence