Note that “disability” is not the same as “impairment”. Just having an injury or disease will not entitle you to disability benefits. Your impairment must cause an inability to work a substantial amount.
To be gainful, your activity must be done in exchange for pay or profit, or be the kind of work usually done for pay or profit, even if you do not realize a profit. This includes work as an employee or self-employment. It includes working “on the books” and it includes working for cash.
To be substantial, your activity must involve doing significant physical or mental activities. There are several tests to determine whether work activity is substantial. If it involves working a full time schedule, or if it pays a certain dollar amount or more (known as the SGA limit), it is usually considered substantial. There are additional tests for self-employment.
Not all work is considered substantial. For example, if your impairments allow you to work three hours per day, but prevent you from working more hours than that, and you work as an employee for less than a certain dollar figure, you are likely to be found disabled. In addition, if you work for someone who pays you more than your work is worth, you might be found disabled even if your income is above the SGA limit.