Monoalphabetic

Part of speech: adjective

Definitions

1. A cipher in which each letter in the plaintext is replaced by a letter with a fixed relationship to it
2. A method of encryption that uses a single alphabet to encode letters, thereby maintaining consistent substitutions
3. A systematic approach to cryptography where each character is mapped to another character without variation throughout the message

Etymology: "Monoalphabetic" is a plain compound: "mono-", from Greek "monos" meaning "single" or "alone", joined to "alphabetic", which traces back through Latin "alphabetum" to the Greek letter names "alpha" and "beta". Put together, it describes a cipher built on one alphabet, where each plain letter is always replaced by the same substitute letter throughout a message. The compound follows the pattern of many scientific and technical coinages that pair a Greek prefix with a familiar base word, so "monoalphabetic" simply reads as "single-alphabet." The contrast it draws is with polyalphabetic ciphers, which switch between several substitution alphabets as a message proceeds. That single-versus-many distinction is what the word exists to name.