skin rash vs urtication

skin rash and urtication both are nouns.

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

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

skin rash: any red eruption of the skin
urtication: 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, urticaria.

skin rash (noun)	urtication (noun)
any red eruption of the skin	a sensation of having been stung by nettles
	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