button ad vs advertisement

button ad and advertisement both are nouns.

Word	Noun	Adjective	Verb	Adverb
button ad	Yes	No	No	No
advertisement	Yes	No	No	No

As nouns, advertisement is a hypernym of button ad; that is, advertisement is a word with a broader meaning than button ad:

button ad: small circular, rectangular, or square advertisement appearing usually at the bottom of a webpage.
advertisement: a public promotion of some product or service

Other hypernyms of button ad include ad, advert, advertising, advertizement, advertizing.

button ad (noun)	advertisement (noun)
small circular, rectangular, or square advertisement appearing usually at the bottom of a webpage.	a public promotion of some product or service