Cipher. (Also cypher).

1(a)A secret way of writing, eps. One in which a set of letters or symbols is used to represent others. A message in sipher. (b) a message written in such a way.( c) a key to such a secret message. Compare CODE1.-DECIPHER
2. The symbol 0, representing zero or no amount.
3. (Derog) a person or thing of no importance. He's a nobody - a mere cipher.