What is the Social Security definition of disability?
To get disability insurance benefits, the worker must have, in addition to the necessary work credits, a physical or mental impairment so severe that it makes him or her unable to do any “substantial” work. Generally, a job that pays about $1,100 or more per month (the figure rises slightly most years) is deemed “substantial” for this purpose.

This is a more stringent definition than that used in federal retirement programs. Thus, a person might be eligible for disability retirement under FERS or CSRS but not under Social Security.