Sometimes called an appliance server, this is a specialized server designed to make instillation and maintenance easier for the user.
The hardware and software need for server appliances is bundled with it so all necessary applications are pre-installed. Once plugged into an existing network the server appliance can begin work immediately.
Almost no configuration is need because the server appliance is designed to run with little or no support.
The end-user does not need to understand any details of the server appliance operating systems, or the commands needed for it, because it offers a narrow selection of network services which makes it easy to operate.
Google Search Appliance, designed for indexing corporate data and returning the results, is an example of a server appliance.
Another example would be vCenter server appliance (vCSA) which is a Linux-based virtual appliance that provides an alternative for organizations not wanting to use Windows vCenter Server, both of which run vSphere, a virtual infrastructure.