nyctalopia vs hypovitaminosis

nyctalopia and hypovitaminosis both are nouns.

Word	Noun	Adjective	Verb	Adverb
nyctalopia	Yes	No	No	No
hypovitaminosis	Yes	No	No	No

As nouns, hypovitaminosis is a hypernym of nyctalopia; that is, hypovitaminosis is a word with a broader meaning than nyctalopia:

nyctalopia: inability to see clearly in dim light; due to a deficiency of vitamin A or to a retinal disorder
hypovitaminosis: any of several diseases caused by deficiency of one or more vitamins

Other hypernyms of nyctalopia include avitaminosis, vision defect, visual defect, visual disorder, visual impairment.

nyctalopia (noun)	hypovitaminosis (noun)
inability to see clearly in dim light; due to a deficiency of vitamin A or to a retinal disorder	any of several diseases caused by deficiency of one or more vitamins