A cipher, or a component of a more complicated cipher, that involves the symbol at each place in the plaintext being (effectively) looked up in a table, and replaced by the substitute symbol found there. Since a cipher must be invertible (for decryption), the table must contain a permutation of the alphabet (compare transposition cipher).

The size of the table can be increased (to strengthen the cipher) by using an extension of the plaintext source. If the table remains constant for the entire plaintext, the substitution is monalphabetic; if it changes with each advance of one symbol position in the plaintext, possibly repeating a fixed schedule, it is polyalphabetic.