The following excerpt is taken directly from the Americans with Disabilities Act (ADA) National Network: Information, Guidance, and Training on the Americans with Disabilities Act.
It is important to remember that in the context of the 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 activities. 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.
It is important to note that the World Health Organization, in 2016, estimated that between 15% to 20% of the world's population has some level of disability. That equates to between 1 billion and 1.4 billion people worldwide.