What is idempotency and why is it important in REST APIs?

Short answer: An operation is idempotent if repeating it has the same effect as doing it once (e.g., PUT). Important for safe retries.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.