The Free On-line Dictionary of Computing (30 December 2018):

expansion card

    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.

   (1998-06-26)