Legally, according to the Americans with Disabilities Act (1990) a disability is "a physical or mental impairment that substantially limits one or more of the major life activities."
Meeting the ADA requirements can sometimes be expensive but the world has become a better place in many ways because of them, and not only for people who have disabilities.
For example, when I was younger wheelchairs were rarely seen in public places, but once curb cuts became required they not only gave more freedom to people using wheelchairs, they also helped people pushing strollers and riding bikes. Back then Sign Language was rarely used in public, but now it has become recognized by Linguists as a real language and many little children have learned to use signs instead of having tantrums. And millions of Americans have had their lives enriched by getting to know people with special needs.