To qualify for social security disability, you must meet a strict criteria.  Part of Social Security's definition of disability is the inability to work any job.  That means that if you are a roofer and can no longer perform this strenuous job due to back problems, but you can sit and perform a less strenuous job like assembling watches on an assembly line, then you are not disabled under Social Security Administration's guidelines. Even working part time shows that you have the ability to work.

In addition the disability must be expected to last for twelve continuous months or be expected to end in death.

Further, the condition must be a "medically determinable impairment." You cannot be determined to be disabled just because you say you are disabled.  You have to have medical documentation to prove your disability.  Your doctor must back up his/her medical diagnosis of your impairment with medical history, diagnostic tests and laboratory results.  (Get my book Documenting Disability for Health Care Providers.) Social Security Disability Blog

To summarize, in order to be found disabled for purposes of the social security administration your condition must be:

1. A medically determinable impairment.
2. Functionally limiting so that all work in precluded.
3. Expected to last for twelve continuous months or be expected to end in death.

Don't let the Social Security Administration's definition of disability deter you from applying if you are unable to work.  If you are denied, they are required to provide you with a written explanation and then you can decide whether or not to appeal their decision.

Social Security Disability