A fixed size random stream that is added to a block cipher to increase randomeness
https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Initialization_vector_(IV) An initialization vector (IV) or starting variable (SV) is a block of bits that is used by several modes to randomize the encryption and hence to produce distinct ciphertexts even if the same plaintext is encrypted multiple times, without the need for a slower re-keying process.