urticaria vs skin rash

urticaria and skin rash both are nouns.

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

As nouns, skin rash is a hypernym of urticaria; that is, skin rash is a word with a broader meaning than urticaria:

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
skin rash: any red eruption of the skin

Other hypernyms of urticaria include efflorescence, rash, roseola, hypersensitivity reaction.

urticaria (noun)	skin rash (noun)
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