plug-in vs circuit board

plug-in and circuit board both are nouns.

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

As nouns, plug-in and circuit board are synonyms defined as:

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

Other synonyms of plug-in include add-in, board, card, circuit card.

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