Brain death defined as irreversible cessation of all brain functions, distinguishing it from vegetative states and cardiopulmonary support. Brain death is the irreversible cessation of all functions of the entire brain, including the brain stem, such that spontaneous respiration and integrated physiological functions cannot be restored. This whole-brain criterion is distinct from a persistent vegetative state-where brain stem activity and some reactions may persist-and from mere mechanical maintenance of cardiopulmonary function. Historical shifts from cardiopulmonary definitions toward whole-brain standards followed advances in life-support technologies and institutional formulations addressing permanence and organ donation.