SSH (Secure Shell) key is a cryptographic key pair used for secure communication and authentication between a client and a server over a network. It consists of a public key, which is shared with the server, and a private key, which remains with the client. The public key encrypts data that can only be decrypted using the private key. SSH keys provide a secure alternative to passwords, as they are harder to crack and eliminate the need to transmit passwords over the network. They are commonly used for remote logins, file transfers, and secure system administration tasks.