In cryptography, an Initial Value or Initialization Vector.
Some ciphers in feedback mode (particularly block ciphers in CBC mode) use previous encryption results (or something else) as basis of next encryption. Thus, to make output look more random, IV is added to the scheme - a number from the hat (or somewhere else - some people use constant IVs, some not) with which the first encryption is XORed with.
This is just an obfuscation scheme, of course - the contents of IV must be sent with ciphertext. | 
