Idempotency in HTTP means that the same request can be performed multiple times with the same effect as if it was executed just once. If you replace current state of some resource with new one, no matter how many times you do so, in the end state will be the same as if you did it just once. To give more concrete example: deleting a user is idempotent because no matter how many times you delete given user by unique identifier, in the end this user will be deleted. On the other hand creating new user is not idempotent because requesting such operation twice will create two users. In HTTP terms here is what RFC 2616: 9.1.2 Idempotent Methods has to say: 9.1.2 Idempotent Methods Methods can also have the property of " idempotence " in that [...] the side-effects of N > 0 identical requests is the same as for a single request. The methods GET, HEAD, PUT and DELETE share this property. Also, the methods OPTIONS and TRACE SHOULD NOT have side effects, and so are inherently i