A file associated with a specific account that contains one or more host keys. Each host key is associated with an SSH server address (IP or hostname) so that the server can be authenticated when a connection is initiated. The user or administrator who makes the first connection to an SSH server is responsible for verifying that the host key presented by that server is the actual key (not a rogue key) before it gets placed in the known hosts file.
Sources:
NISTIR 7966