nyctalopia vs visual impairment

nyctalopia and visual impairment both are nouns.

Word	Noun	Adjective	Verb	Adverb
nyctalopia	Yes	No	No	No
visual impairment	Yes	No	No	No

As nouns, visual impairment is a hypernym of nyctalopia; that is, visual impairment 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
visual impairment: impairment of the sense of sight

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

nyctalopia (noun)	visual impairment (noun)
inability to see clearly in dim light; due to a deficiency of vitamin A or to a retinal disorder	impairment of the sense of sight