For the purpose of the exception to the penalty for early distributions from retirement accounts, the IRS says in Pub 590-B:
You are considered disabled if you can furnish proof that you can't do any substantial gainful activity because of your physical or mental condition. A physician must determine that your condition can be expected to result in death or to be of long, continued, and indefinite duration.
There is no requirement that you be receiving SSDI. SSDI has a somewhat different definition of "disabled."
