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, like the Caesar cipher, where each letter is shifted by a fixed number, or more complex, using a random arrangement of letters. Substitution ciphers are a basic form of cryptography, which is the practice of securing information through encoding.