double vision vs vision defect

double vision and vision defect both are nouns.

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

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

Other hypernyms of double vision include visual defect, visual disorder, visual impairment.

double vision (noun)	vision defect (noun)
visual impairment in which an object is seen as two objects	impairment of the sense of sight