Impairment, disorder and disability are familiar terms introduced by the World Health Organization (WHO) in 1980. In the WHO classifications, impairment is defined as an abnormality of a structure or function; disability is the functional consequence of impairment; and handicap is the social consequence of impairment.

The Department of Rehabilitation Services embraces a slightly different model from the NIH National Center for Medical Rehabilitation Research, which describes five levels of function.