Challenge Handshake Authentication Protocol (CHAP): A network authentication protocol where the server challenges a client to prove its identity. The server sends a unique challenge string, and the client responds with a value obtained by hashing the challenge with its password. If the values match, authentication is successful. CHAP provides more security than password-based authentication because the password is not sent over the network. It also periodically re-authenticates to protect against session hijacking or replay attacks.