circuit board vs add-in

circuit board and add-in both are nouns.

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

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

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

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

circuit board (noun)	add-in (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