<hardware> A circuit board which can be plugged into one of a computer's expansion slots to provide some optional extra facility such as additional RAM, disk controller, coprocessor, graphics accelerator, communication device or some special-purpose interface.
Different computers have different standards for the cards they accept, e.g. PCI.
Try this search on Wikipedia, OneLook, Google
Nearby terms: expanded memory manager « expanded memory page frame « Expanded Memory Specification « expansion card » expansion slot » expect » eXperimental LISP