A limit in a range of major life activities. This includes activities like seeing, hearing, walking and tasks like thinking and working. Because different programs may have different disability standards, please check the program you're interested in for its disability standards.
The list of activities mentioned above isn't exhaustive. A legal definition of disability can be found here: http://www.ada.gov/pubs/ada.htm. For the EEOC ADA Amendments Act regulations, and related resources (PDF), see http://www.gpo.gov/fdsys/pkg/FR-2011-03-25/pdf/2011-6056.pdf.