A public key used for authenticating a host in the SSH protocol to hosts that want to communicate with it (each host also generally has its own private host key). Some hosts may have more than one host key (e.g., one for each algorithm). Host keys are used for authenticating hosts (machines) themselves, not users or accounts, whereas identity keys and authorized keys relate to authenticating users/accounts and authorizing access to accounts on hosts.
Sources:
NISTIR 7966