direct mail vs advertisement

direct mail and advertisement both are nouns.

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

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

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

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

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