Initialization vector
The non-secret starting value that makes the same message encrypt to different ciphertext twice running.
An initialization vector is fed in alongside the key and need not be kept secret — it is normally sent in the clear ahead of the ciphertext. Without one, encryption would be deterministic, and an attacker could tell when the same message was sent again.
The requirements differ between modes, and this is where the mistakes happen. CBC requires the vector to be unpredictable: if the attacker can guess the next IV, guesses about the plaintext can be verified. CTR and GCM only require it to be unique, but there reuse is catastrophic — in GCM a repeated nonce leaks the authentication key itself. WEP's 24-bit IV is the textbook example of what happens when the space runs out.