plug-in vs add-in

plug-in and add-in both are nouns.

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

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

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

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