plug-in vs CPU board

plug-in and CPU board both are nouns.

Word	Noun	Adjective	Verb	Adverb
plug-in	Yes	No	No	No
CPU board	Yes	No	No	No

As nouns, CPU board is a hyponym of plug-in; that is, CPU board is a word with a more specific, narrower meaning than plug-in:

plug-in: a printed circuit that can be inserted into expansion slots in a computer to increase the computer's capabilities
CPU board: the main circuit board for a computer

Other hyponyms of plug-in include mother board, PC board.

plug-in (noun)	CPU board (noun)
a printed circuit that can be inserted into expansion slots in a computer to increase the computer's capabilities	the main circuit board for a computer
a software module that extends or enhances the capabilities of an existing application