A transposition cipher is a method of encryption that rearranges the letters of a message to create a coded version. Instead of changing the letters themselves, it alters their positions based on a specific system or key. This means that the original message can be reconstructed if the correct method is known. For example, in a simple transposition cipher, the message "HELLO" might be rearranged to "OLEHL." This type of cipher is often used in combination with other encryption methods to enhance security. It is a fundamental concept in the field of cryptography, which studies techniques for secure communication.