direct mail vs ad

direct mail and ad both are nouns.

Word	Noun	Adjective	Verb	Adverb
direct mail	Yes	No	No	No
ad	Yes	No	No	No

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

direct mail: advertising sent directly to prospective customers via the mail
ad: a public promotion of some product or service

Other hypernyms of direct mail include advert, advertisement, advertising, advertizement, advertizing.

direct mail (noun)	ad (noun)
advertising sent directly to prospective customers via the mail	a public promotion of some product or service