top billing vs ad

top billing and ad both are nouns.

Word	Noun	Adjective	Verb	Adverb
top billing	Yes	No	No	No
ad	Yes	No	No	No

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

top billing: the advertisement of a star's name at the top of a theatrical poster
ad: a public promotion of some product or service

Other hypernyms of top billing include advert, advertisement, advertising, advertizement, advertizing.

top billing (noun)	ad (noun)
the advertisement of a star's name at the top of a theatrical poster	a public promotion of some product or service