A device that is dedicated to a specific function in contrast to a general-purpose computer. The hardware comes pre-installed with its operating system and application. Many consider the router to be the first network appliance.A "software appliance" is the software alone, which must be installed in the computer. Following is a list of common hardware appliances. See virtual appliance. caching appliance digital appliance email appliance firewall appliance Internet appliance Internet server appliance network appliance server appliance.