A Disability Determination is by following a 

five-step process:

1.  Are you working?

2.  Is your medical condition “severe”?

3.  Does your impairment(s) meet or medically   

     equal a Listing?

4.  Can you do the work you did before?

5.  Can you do any other type of work?

SSA defines disability as the inability to engage in any substantial gainful activity (SGA) by reason of any medically determinable physical or mental impairment(s) 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 12 months. 

SSA maintains a "Listing of Medical Impairments".  

If your medical condition, or its equivalent is on SSA's Listing of Impairments then you're generally considered disabled and therefore eligible to receive SSA benefits.  If your condition is not on the list, you may still be eligible under other SSA guidelines. 

SSA determines whether someone can do the work they used to do – their “past relevant work.”  or whether they can do any other work in the national economy, considering that person’s age, educational background, work experience and, of course, impairments. 

Finally if SSA finds that they are not able to engage in other work as a result of their impairments, that person will be entitled to benefits, possibly including back benefits.