Idempotency (API)
The guarantee that retrying the same API request once or five times has the same effect as making it once.
Networks fail mid-request; clients retry. Idempotency keys let a write API recognize a retry of an already-processed request and return the original result instead of, say, creating a second invoice.
It's table stakes for money-adjacent APIs — and a quick tell of API maturity when you're evaluating a platform to build on.
In the product: API & Developers. Related: Webhook