A substitution cipher is a method of encoding messages by replacing each letter in the plaintext with another letter. For example, in a simple substitution cipher, the letter 'A' might be replaced with 'D', 'B' with 'E', and so on. This creates a coded message that can only be understood by someone who knows the specific substitutions used. These ciphers can be simple or complex, depending on how many letters are substituted and the rules applied. They are a basic form of cryptography, which is the study of secure communication. Famous examples include the Caesar cipher, named after Julius Caesar, who used it to protect military messages.