Substitution cipher

From the Quicksilver Metaweb.

A substitution cipher is one where each letter of a plaintext message is replaced by some other symbol. Most frequently the symbols used for replacement are letters of the alphabet, but they can also be numbers, groups of letters, or other symbols entirely (for instance the stick figures which play a part in the Sherlock Holmes story The Adventures Of The Dancing Men).

A simple, or monoalphabetic, substitution cipher (such as a Caesar cipher) is one where each occurrence of a letter in the plaintext message is replaced by the same symbol. A polyalphabetic substitution cipher is one where different occurrences of a letter in the plaintext messages may be replaced by different symbols - the Vigenère cipher is an example.