nyctalopia vs visual defect

nyctalopia and visual defect both are nouns.

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

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

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

nyctalopia (noun)	visual defect (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