eruption vs skin rash

eruption and skin rash both are nouns.

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

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

eruption (noun)	skin rash (noun)
the sudden occurrence of a violent discharge of steam and volcanic material	any red eruption of the skin
a sudden very loud noise
the emergence of a tooth as it breaks through the gum
a sudden violent spontaneous occurrence (usually of some undesirable condition)
(of volcanoes) pouring out fumes or lava (or a deposit so formed)
symptom consisting of a breaking out and becoming visible