nyctalopia vs vision defect

nyctalopia and vision defect both are nouns.

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

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

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

nyctalopia (noun)	vision 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