melanosis vs skin disease

melanosis and skin disease both are nouns.

Word	Noun	Adjective	Verb	Adverb
melanosis	Yes	No	No	No
skin disease	Yes	No	No	No

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

melanosis: a condition characterized by abnormal deposits of melanin (especially in the skin)
skin disease: a disease affecting the skin

Other hypernyms of melanosis include disease of the skin, skin disorder.

melanosis (noun)	skin disease (noun)
a condition characterized by abnormal deposits of melanin (especially in the skin)	a disease affecting the skin