rash vs heat rash

rash and heat rash both are nouns.

Word	Noun	Adjective	Verb	Adverb
rash	Yes	Yes	No	No
heat rash	Yes	No	No	No

As nouns, heat rash is a hyponym of rash; that is, heat rash is a word with a more specific, narrower meaning than rash:

Other hyponyms of rash include miliaria, prickly heat, hives, nettle rash, urticaria, urtication.

rash (noun)	heat rash (noun)
a series of unexpected and unpleasant occurrences	obstruction of the sweat ducts during high heat and humidity
any red eruption of the skin

rash (adjective)	heat rash (adjective)
marked by defiant disregard for danger or consequences
imprudently incurring risk