[agrafix]

Usually I’ve seen it defined as that the intended effect happens at most once. e.g. see https://developer.mozilla.org/en-US/docs/Glossary/Idempotent

[aastronaut]

From your link:

> An HTTP method is idempotent if an identical request can be made once or several times in a row with the same effect while leaving the server in the same state.

Which is according to what OP wrote.

I wouldn't translate this into "the intended effect happens at most once", as that behavior includes the effect of the transport. "at most once", "at least once" and "exactly once" are messaging behaviors after all, and idempotency is a method to convert an "at least once"-messaging-effect into an "exactly once"-change-effect.