Short Definition
Idempotency means repeating an operation has the same effect as performing it once.
Intuition
If a request is retried after a network failure, idempotency prevents the action from happening twice.
Technical Definition
An operation is idempotent if applying it multiple times produces the same resulting state as applying it once, assuming equivalent inputs and state constraints.
Example
A payment API may use an idempotency key so retrying a charge request does not bill twice.
Common Misunderstandings
Idempotency does not mean an operation has no side effects.
It must be designed around identity, state, and failure modes.