What does idempotency stand for in REST?

Idempotency guarantees clients that repeating the same request has the same effect as making a request just once.

DELETE method is idempotent. This implies that the server must return response code 200 OK even if the server deleted the resource in a previous request. For DELETE it means that the server should keep track of previous delete requests (or deleted resources).

Idempotent methods:

GET, HEAD, OPTIONS,PUT, DELETE

Non idempotent methods:

POST, PATCH