It is helpful to think of abilities and disabilities on a continuum. A disability does not have an on/off switch.

We all live on a continuum of being able and unable to perform tasks.

Spectrum on disabilities on a continuum. Not able on the left. Able on the right.

Read print,
Use a mouse,
Write with pen or pencil,
Communicate verbally,
Tune out distraction,
Manage physical/mental health

People live on a spectrum from totally blind, legally blind, see color, need glasses, and a few lucky people have 20/20 vision. As people age through life they drift farther away from the "able" side.

What is the definition of disability under the ADA?

It is important to remember that in the context of the Americans with Disabilities Act (ADA), “disability” is a legal term rather than a medical one. Because it has a legal definition, the ADA’s definition of disability is different from how disability is defined under some other laws.

The ADA defines a person with a disability as a person who has a physical or mental impairment that substantially limits one or more major life activity. This includes people who have a record of such an impairment, even if they do not currently have a disability. It also includes individuals who do not have a disability but are regarded as having a disability. The ADA also makes it unlawful to discriminate against a person based on that person’s association with a person with a disability. Source