Challenge-response authentication: A method of authentication in which a server presents a question or challenge to a user or system seeking access, who must then provide a valid answer or response. This type of authentication is often used in scenarios where passwords alone are deemed insufficiently secure. The challenge is typically a random number, and the response is the correct encryption of this number using a shared key. By ensuring that the response is correct, the system verifies the identity of the user or system, thereby enhancing the security of the access process.