hard drug vs heroin

hard drug and heroin both are nouns.

Word	Noun	Adjective	Verb	Adverb
hard drug	Yes	No	No	No
heroin	Yes	No	No	No

As nouns, heroin is a hyponym of hard drug; that is, heroin is a word with a more specific, narrower meaning than hard drug:

hard drug: a narcotic that is considered relatively strong and likely to cause addiction
heroin: a narcotic that is considered a hard drug; a highly addictive morphine derivative; intravenous injection provides the fastest and most intense rush

Other hyponyms of hard drug include cocain, cocaine, diacetylmorphine.

hard drug (noun)	heroin (noun)
a narcotic that is considered relatively strong and likely to cause addiction	a narcotic that is considered a hard drug; a highly addictive morphine derivative; intravenous injection provides the fastest and most intense rush