1. In cryptography, a cipher is a cryptographic system (see cryptography) in which units of plain text of regular length, usually letters, are arbitrarily transposed (see transposition cipher) or substituted (see substitution cipher) according to a predetermined code, or a message written or transmitted in such a system. See also Caesar cipher and Beal ciphers.
2. In mathematical nomenclature, a cipher is the symbol (0) for zero.