Rabbit Stream Cipher Initialization Vector. See RFC 4503 3.2. Initialization Vector (page 5).
It is possible to run Rabbit without the IV setup. However, in this case, the generator must never be reset under the same key, since this would destroy its security (for a recent example, see 4). However, in order to guarantee synchronization between sender and receiver, ciphers are frequently reset in practice. This means that both sender and receiver set the inner state of the cipher back to a known value and then derive the new encryption state using an IV. If this is done, it is important to make sure that no IV is ever reused under the same key.