Idempotency
In plain terms
A property where doing an operation multiple times has the same effect as doing it once.
Going deeper
Idempotent operations are safe to retry, which matters when networks fail and you're unsure if a request went through. Setting your address to a value is idempotent; incrementing a counter is not. APIs lean on it to handle retries without double-charging or duplicating work.
Under the hood
Critical for safe retries in distributed systems; achieved with idempotency keys, conditional writes, or naturally idempotent operations (PUT/DELETE vs. POST).
Examples
PUT /user/42 setting a value; payment APIs using an idempotency-key header; Kubernetes reconciliation.
Think of it like...
Like pressing a floor button in an elevator: mashing it ten times still takes you to that one floor exactly once.