The ADA has a three-part definition of "disability." This definition taken from the Rehabilitation Act reflects the specific types of discrimination experienced by people with disabilities. Accordingly, it is not the same as the definition of disability in other laws, such as state workers' compensation laws or other federal or state laws that provide benefits for people with disabilities and disabled veterans.
Under the ADA, an individual with a disability is a person who:
For information regarding documentation of disability see Enforcement Guidance: Reasonable Accommodation and Undue Hardship under the Americans with Disabilities Act, specifically questions 6, 7, 8.