appurtenance vs add-on

appurtenance and add-on both are nouns.

Word	Noun	Adjective	Verb	Adverb
appurtenance	Yes	No	No	No
add-on	Yes	No	No	No

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

appurtenance and add-on: a supplementary component that improves capability

Other synonyms of appurtenance include accessory, supplement.

appurtenance (noun)	add-on (noun)
a supplementary component that improves capability	a component that is added to something to improve it
equipment consisting of miscellaneous articles needed for a particular operation or sport etc.	a supplementary component that improves capability
	a software module that extends or enhances the capabilities of an existing application