add-in vs mother board

add-in and mother board both are nouns.

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

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

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

Other hyponyms of add-in include CPU board, PC board.

add-in (noun)	mother 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