visual defect vs impairment

visual defect and impairment both are nouns.

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

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

visual defect: impairment of the sense of sight
impairment: the condition of being unable to perform as a consequence of physical or mental unfitness

Other hypernyms of visual defect include disability, disablement, handicap.

visual defect (noun)	impairment (noun)
impairment of the sense of sight	damage that results in a reduction of strength or quality
	the act of making something futile and useless (as by routine)
	the occurrence of a change for the worse
	the condition of being unable to perform as a consequence of physical or mental unfitness
	a symptom of reduced quality or strength