You may have an official definition for the jurisdiction you are in.
E.g. in the UK the guidance to the Equality Act 2010 states:
> A person has a disability for the purposes of the Act if he or
she has a physical or mental impairment and the impairment has a
substantial and long-term adverse effect on his or her ability to carry
out normal day-to-day activities.
This is part of
$1.
Hopefully you have similar local guidance. More practically if there is a local charity or society that supports visually impaired people they may be able to give you guidance.