Property where performing the same operation multiple times produces the same result as doing it once. Like pressing an elevator button repeatedly - the elevator still only comes once.
Payment APIs use idempotency so if a customer accidentally clicks 'Pay' twice, they're only charged once. The second identical request is safely ignored.
Idempotency is an API design property, not a standalone cloud service. Some platforms offer features that help implement it (for example, AWS API Gateway can enforce idempotency for certain requests), but in most clouds you implement idempotency in your application using idempotency keys, request deduplication, and transactional storage.