It is a a definition given by the law to make sure that only dead people will donate organs. The machine tells you whether a patient is brain dead or not. This is not science fiction, but daily life for organ transplantations.
The machine tells you whether a patient is brain dead or not.
No such machine exists. Dead people can't be used to donate most organs (I think perhaps corneas are one of a few exceptions). Your heart must be beating and you must be breathing for organs to be useful. If they wait for even 2 minutes after those functions stop, the organs receive too much damage for a transplant.