A substitution cipher is a method of encoding messages by replacing each letter in the plaintext with a different letter or symbol. 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 be deciphered only by someone who knows the specific substitutions used. These ciphers can be simple, like the Caesar cipher, where each letter is shifted by a fixed number, or more complex, using random substitutions. While substitution ciphers can provide a basic level of security, they are vulnerable to frequency analysis, where patterns in the encoded text can reveal the original message.