For Social Security purposes, Congress defined "disability" as 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 twelve months.

It is helpful if your doctor believes you meet this "disability" definition.  The Social Security decision is based on many complex medical, vocational, and legal factors.  Even if you are found disabled by one agency, you may not qualify for Social Security.  For example, an individual may be entitled to a VA Disability Pension, or Long-Term Disability or Worker's Compensation, but not to Social Security benefits.