add-on vs plug-in

add-on and plug-in both are nouns.

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

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

add-on and plug-in: a software module that extends or enhances the capabilities of an existing application

add-on (noun)	plug-in (noun)
a component that is added to something to improve it	a printed circuit that can be inserted into expansion slots in a computer to increase the computer's capabilities
a supplementary component that improves capability	a software module that extends or enhances the capabilities of an existing application
a software module that extends or enhances the capabilities of an existing application