add-in vs plug-in

add-in and plug-in both are nouns.

Word	Noun	Adjective	Verb	Adverb
add-in	Yes	No	No	No
plug-in	Yes	No	No	No

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

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

add-in (noun)	plug-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
	a software module that extends or enhances the capabilities of an existing application