A continuously changing number used in combination with a secret key to encrypt data. Initialization vectors (IVs) are used to prevent a sequence of text that is identical to a previous sequence from producing the same exact ciphertext when encrypted. For example, packets have address fields that are generally fixed in location within the header of the packet. If attackers view the same encrypted data over and over, it provides them with clues to interpret their original values. See nonce.
Used in cryptography to ensure that an encryption mechanism, such as a stream cipher or a block cipher in a streaming mode, generates a unique stream that is independent of all other streams encrypted with the same key without reapplying the (computationally expensive) cryptographic keying process. The Initialization Vector must be known by the receiver and can be exchanged as part of the session setup or transmitted independently. Ferguson, N, Schneier, B. Practical Cryptography. New York, NY: John Wiley & Sons, 2003.