idempotent method
GET, HEAD, OPTIONS, TRACE, PUT, DELETE
the intended effect on the server of multiple identical requests is the same as for a single such request
A method is idempotent if issuing the same request twice (or ten times) has the same intended effect on the server as issuing it once. PUT and DELETE are idempotent (PUT the same body twice → same final state; DELETE twice → still deleted). POST and PATCH are not.
Why it matters: a client or proxy may automatically retry an idempotent request after a network failure without risking a double-effect. This is the property that makes HTTP survivable on unreliable networks.
How it is read
Mathematics
tradition
A function f is idempotent if f(f(x)) = f(x) — applying it again changes nothing. Rounding, absolute value, and "set to 5" are idempotent; "add 1" is not.
RFC 9110 §9.2.2
spec
"A request method is considered idempotent if the intended effect on the server of multiple identical requests with that method is the same as the effect for a single such request." Note intended effect — the server's actual response codes may differ (the second DELETE gets 404), but the resource state is the same. Explicitly permits automatic retry of idempotent requests.