add-in vs circuit board

add-in and circuit board both are nouns.

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

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

add-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 add-in include board, card, circuit card, plug-in.

add-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