The Official Definition of Disability:
Straight from the Social Security Act, with minor editing to make it more readable, here’s the definition of disability for non-blind adults (there are special rules for people who are legally blind):
The inability to engage in any substantial gainful activity by reason of any medically determinable physical or mental impairment which can be expected to result in death or which has lasted or can be expected to last for a continuous period of not less than 12 months....
An individual shall be determined to be under a disability only if his physical or mental impairment or impairments are of such severity that he is not only unable to do his previous work but cannot, considering his age, education, and work experience, engage in any other kind of substantial gainful work which exists in the national economy, regardless of whether such work exists in the immediate area in which he lives, or whether a specific job vacancy exists for him, or whether he would be hired if he applied for work. For purposes of the preceding sentence (with respect to any individual), “work which exists in the national economy” means work which exists in significant numbers either in the region where such individual lives or in several regions of the country.
In determining whether an individual’s physical or mental impairment or impairments are of a sufficient medical severity that such impairment or impairments could be the basis of eligibility under this section, the Commissioner of Social Security shall consider the combined effect of all of the individual’s impairments without regard to whether any such impairment, if considered separately, would be of such severity. If the Commissioner of Social Security does find a medically severe combination of impairments, the combined impact of the impairments shall be considered throughout the disability determination process.
An individual shall not be considered to be disabled ... if alcoholism or drug addiction would (but for this subparagraph) be a contributing factor material to the Commissioner’s determination that the individual is disabled.
For purposes of this subsection, a “physical or mental impairment” is an impairment that results from anatomical, physiological, or psychological abnormalities which are demonstrable by medically acceptable clinical and laboratory diagnostic techniques.
An individual shall not be considered to be under a disability unless he furnishes such medical and other evidence of the existence thereof as the Commissioner of Social Security may require. An individual’s statement as to pain or other symptoms shall not alone be conclusive evidence of disability as defined in this section; there must be medical signs and findings, established by medically acceptable clinical or laboratory diagnostic techniques, which show the existence of a medical impairment that results from anatomical, physiological, or psychological abnormalities which could reasonably be expected to produce the pain or other symptoms alleged and which, when considered with all evidence required to be furnished under this paragraph (including statements of the individual or his physician as to the intensity and persistence of such pain or other symptoms which may reasonably be accepted as consistent with the medical signs and findings), would lead to a conclusion that the individual is under a disability. Objective medical evidence of pain or other symptoms established by medically acceptable clinical or laboratory techniques (for example, deteriorating nerve or muscle tissue) must be considered in reaching a conclusion as to whether the individual is under a disability.
Notwithstanding any other provision of this title, any physical or mental impairment which arises in connection with the commission by an individual (after the date of the enactment of this paragraph) of an offense which constitutes a felony under applicable law and for which such individual is subsequently convicted, or which is aggravated in connection with such an offense (but only to the extent so aggravated), shall not be considered in determining whether an individual is under a disability.