Original: A hostname is the unique name by which a network-attached device (which could consist of a computer, file server, network storage device, fax machine, copier, cable modem, etc.) is known on a network.
Simplified: A hostname (also known as, a sitename) is the unique name by which a network device (which may be a computer, file server, network storage device, fax machine, copier, cable modem, etc.) is defined and known on a network.