The best definition of a Host Scan is:

When one host on the network attaches to a network printer-scanner device and is able to exploit the printer-scanner to download all material scanned on the device.

When one host on the network is infected with malware and the malware scans all of the files resident on the infected system looking for exploits.

When several hosts on the network send a high volume of traffic to a single host to try to exhaust the resources of the host.

When one host on the network send ping packets or other packet types to multiple other devices on the network to determine which hosts are available for further exploit.