policeman vs adult male

policeman and adult male both are nouns.

Word	Noun	Adjective	Verb	Adverb
policeman	Yes	No	No	No
adult male	Yes	No	No	No

As nouns, adult male is a hypernym of policeman; that is, adult male is a word with a broader meaning than policeman:

Other hypernyms of policeman include man, officer, police officer.

policeman (noun)	adult male (noun)
a male member of a police force	an adult person who is male (as opposed to a woman)