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

daughterboard
daughtercard

    (Or "daughter board", "daughtercard", "daughter
   card") A printed circuit board that connects to the
   motherboard.  The daughterboard is typically smaller than
   the motherboard.

   A daughterdboard often adds to or supports the main functions
   of the motherboard, unlike an expansion card which
   provides some new function.  For example, a post-release
   hardware modification might be released as a daughterboard for
   soldering onto the motherboard.

   (2004-09-28)