Wiktionary
monoalphabetic
a. (context cryptography English) Of a substitution cipher, using the same fixed mappings from plaintext to cipher letters across the entire text.
Usage examples of "monoalphabetic".
This would have the effect of enciphering one letter into another in a monoalphabetic substitution.
Their solution consists in large measure of breaking down the cipher until the method for solving monoalphabetic substitutions can be applied.