extension vs add-on

extension and add-on both are nouns.

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

As nouns, add-on is a hypernym of extension; that is, add-on is a word with a broader meaning than extension:

Other hypernyms of extension include addition, improver.

As nouns, add-on is a hypernym of extension; that is, add-on is a word with a broader meaning than extension:

Other hypernyms of extension include addition, improver.

extension (noun)	add-on (noun)
an educational opportunity provided by colleges and universities to people who are not enrolled as regular students	a component that is added to something to improve it
act of stretching or straightening out a flexed limb	a supplementary component that improves capability
act of expanding in scope; making more widely available	a software module that extends or enhances the capabilities of an existing application
an addition that extends a main building
an addition to the length of something
an additional telephone set that is connected to the same telephone line
amount or degree or range to which something extends
the ability to raise the working leg high in the air
the most direct or specific meaning of a word or expression; the class of objects that an expression refers to
a string of characters beginning with a period and followed by one or more letters; the optional second part of a PC computer filename
the spreading of something (a belief or practice) into new regions
a mutually agreed delay in the date set for the completion of a job or payment of a debt