What is the X in X-mas?
a small printed circuit board that attaches to a larger one
(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.