circular vs ad

circular and ad both are nouns.

Word	Noun	Adjective	Verb	Adverb
circular	Yes	Yes	No	No
ad	Yes	No	No	No

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

circular: an advertisement (usually printed on a page or in a leaflet) intended for wide distribution
ad: a public promotion of some product or service

Other hypernyms of circular include advert, advertisement, advertising, advertizement, advertizing.

circular (noun)	ad (noun)
an advertisement (usually printed on a page or in a leaflet) intended for wide distribution	a public promotion of some product or service

circular (adjective)	ad (adjective)
having the shape or form of a circle
describing a circle; moving in a circle