ichthyosis vs xeroderma

ichthyosis and xeroderma both are nouns.

Word	Noun	Adjective	Verb	Adverb
ichthyosis	Yes	No	No	No
xeroderma	Yes	No	No	No

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

ichthyosis: any of several congenital diseases in which the skin is dry and scaly like a fish
xeroderma: a mild form of ichthyosis characterized by abnormal dryness and roughness of the skin

Other hyponyms of ichthyosis include xerodermia.

ichthyosis (noun)	xeroderma (noun)
any of several congenital diseases in which the skin is dry and scaly like a fish	a mild form of ichthyosis characterized by abnormal dryness and roughness of the skin