add-in vs circuit card

add-in and circuit card both are nouns.

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

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

add-in and circuit card: 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 board, plug-in.

add-in (noun)	circuit card (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