skin rash vs urticaria

skin rash and urticaria both are nouns.

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

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

skin 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 skin rash include heat rash, miliaria, prickly heat, hives, nettle rash, urtication.

skin rash (noun)	urticaria (noun)
any red eruption of the skin	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