Brain death occurs when someone's brain stops functioning, even though their heart may be kept beating using a machine.
brain death in British English
noun
irreversible cessation of respiration due to irreparable brain damage, even though the heart may continue beating with the aid of a mechanical ventilator: widely considered as the criterion of death
Also called: cerebral death
brain death in American English
a condition in which all vital functions of the brain are determined to have irreversibly ceased: variously accepted as a legal concept of death