Monoalphabetic, monoliteral simple substitution systems.
We use this term to refer to systems in which a symbol or letter is replaced by a different one (monoliteral)—specifically, the same replacement is always used, meaning a single substitution alphabet (monoalphabetic) is employed. Essentially, we have two alphabets to use for encryption or decryption. The encryption (and decryption) process is simple: we take each letter of the message to be encrypted, locate its position, and replace it with the corresponding letter from the other alphabet. We will refer to the alphabet used as the base as the "plaintext alphabet" and the other as the "ciphertext alphabet."