An adult is disabiled if they are unable to do any substantial work because of their medical condition(s); and their medical condition(s) have lasted, or are expected to last for a continuous period of at least 12 months or to result in death.
A child is disabled if they have a physical or mental condition that very seriously limits their activities and has lasted, or is expected to last for a continuous period of at least 12 months or to result in death.
Your disability must prevent you from engaging in your previous employment or like work, and prevent you from any other kind of substantial gainful work which exists in the national economy. It does not matter if such work exists, just that you could do it if it was available.