According to the ADA, an “individual with a disability” is defined as anyone with a physical or mental impairment that substantially limits a major life activity.*  

This includes but is not limited to:

This also includes impairment of major bodily functions including the functioning of the immune system, special sense organs, and skin, normal cell growth, digestive system , urinary  system, bowel, bladder, neurological, brain, respiratory, circulatory, cardiovascular, endocrine, hemic, lymphatic, musculoskeletal, and reproductive functions, as well as the operation of an individual organ within a body system. This definition also includes psychological and learning disabilities, and chronic health impairments.

For more information and a more detailed definition, please visit: 

U.S. Department of Justice Americans with Disabilities Act Homepage

* The given definition has been updated to include revisions from the ADA Amendments Act, January 1, 2009.