A printed circuit board that plugs into an expansion slot in a computer to provide some extra facility. The sockets normally connect to a bus, and the type of connector and the use to which each contact is put are strictly defined to ensure compatibility between the card and the computer. Cards are available that provide extra memory, communications interfaces, sound I/O capabilities, device interfaces to extra disks, for instance, or perhaps extra processors in multiprocessor systems. See also PCMCIA.