rash vs urticaria

rash and urticaria both are nouns.

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

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

rash: any red eruption of the skin
urticaria: an itchy skin eruption characterized by weals with pale interiors and well-defined red margins; usually the result of an allergic response to insect bites or food or drugs

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

rash (noun)	urticaria (noun)
a series of unexpected and unpleasant occurrences	an itchy skin eruption characterized by weals with pale interiors and well-defined red margins; usually the result of an allergic response to insect bites or food or drugs
any red eruption of the skin

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