Cipher for Telegraphic Correspondence — a code book used by Union General Joseph Hooker’s code clerk

In cryptography, a codebook is a document used for implementing a code. A codebook contains a lookup table for coding and decoding; each word or phrase has (one or more) strings which replace it. To decipher messages written in code, identical copies of the codebook must be available at either end. The distribution and physical security of codebooks presents a special difficulty in the use of codes, compared to the secret information used in ciphers, the key, which is typically much shorter.

NSA documents sometimes use "codebook" to mean 'block cipher'; compare their use of "combiner-type algorithm" to mean 'stream cipher'.