Ciphertext-only refers to a type of cryptanalysis where the attacker has access only to the encrypted data, known as ciphertext, without any corresponding plaintext or key. The goal is to deduce information about the original message or the encryption method used. In this scenario, the attacker analyzes patterns, frequencies, and other characteristics of the ciphertext to uncover potential vulnerabilities. This method is often used against classical ciphers, where the lack of complexity can make it easier to break the encryption without additional information.