Under the ADA and the related (Federal Employees) Rehabilitation act, disabled means suffering an impairment that substantially limits a major life function such as walking, sleeping, working etc. The Democrats in Congress modified existing Court interpretations (Sutton and Williams) to expand the definition allowing more people to claim protected status under those two acts and thus sue under the twin theories of a "failure to accommodate" or "disparate treatment based on status".
having litigated dozens of such cases there are as many legitimate cases where employers either failure to accommodate or subject the individual to disparate treatment as there are cases where lousy employees try to avoid the consequences of their poor performance. I note that the USPS has something nearing 30% of its workers engaged in Limited (due to on the job impairments) or Light (due to impairments not involving workers compensation implications) duty. Many of those people are not "disabled" within the meaning of the two federal laws but are being accommodated due to alleged impairments
Its a huge racket and we need to be far more careful in evaluating such claims and far more rigorous in rooting out fraud