moon blindness vs vision defect

moon blindness and vision defect both are nouns.

Word	Noun	Adjective	Verb	Adverb
moon blindness	Yes	No	No	No
vision defect	Yes	No	No	No

As nouns, vision defect is a hypernym of moon blindness; that is, vision defect is a word with a broader meaning than moon blindness:

moon blindness: 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 moon blindness include avitaminosis, hypovitaminosis, visual defect, visual disorder, visual impairment.

moon blindness (noun)	vision defect (noun)
recurrent eye inflammation in horses; sometimes resulting in blindness	impairment of the sense of sight
inability to see clearly in dim light; due to a deficiency of vitamin A or to a retinal disorder