sightlessness vs vision defect

sightlessness and vision defect both are nouns.

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

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

Other hypernyms of sightlessness include visual defect, visual disorder, visual impairment.

sightlessness (noun)	vision defect (noun)
the state of being blind or lacking sight	impairment of the sense of sight