Disability refers to the social effects of physical or mental impairment. This definition, known as the 'social model' of disability, makes a clear distinction between the impairment itself (such as a medical condition that makes a person unable to walk) and the disabling effects of society in relation to that impairment. In simple terms, it is not the inability to walk that prevents a person entering a building unaided but the existence of stairs that are inaccessible to a wheelchair-user. In other words, 'disability' is socially constructed. The 'social model' is often contrasted with the 'medical model' which sees 'disability' as synonymous with 'impairment.' - Wikipedia at: http://en.wikipedia.org/wiki/Disabilities#Defining_Disabilities  Marie Deatherage

Disability also has a legal context.  A disability as defined by federal law is a condition that interferes with an individual's ability to perform one or more activities of everyday living. For example, locomotion (indoors and going outside), getting dressed, communicating with others.

http://www.eeoc.gov/policy/ada.html