For tax purposes, disability is only relevant for children. Even then, a child's disability doesn't affect tax filing until he turns 19 (24 if a full time student). Then his disability allows him to continue to be considered a "Qualifying Child" for tax purposes regardless of age.
But to answer your question, No specific medical diagnosis constitutes disabled to tax purposes.
“The term ‘disability’ means, with respect to an individual –
(a) a physical or mental impairment that substantially limits one or more of the major life activities of such individual;
(b) a record of such impairment; or
(c) being regarded as having such an impairment.”
For the IRS, disabled means -
"an individual shall be considered to be disabled if he is unable 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 to be of long-continued and indefinite duration. An individual shall not be considered to be disabled unless he furnishes proof of the existence thereof in such form and manner as the Secretary may require. "
I believe the substance of that paragraph is that it is your Doctor's decision
