Brain death is the state where there is global ischaemia of the brain while the body is maintained through artificial ventilation and the heart continues to function. In the era of modern medicine, the state of brain death has been taken to be synonymous with death.
The diagnosis of brain death requires at least the following:
1. widespread cortical destruction - deep coma, unresponsive to all forms of stimulation.
2. brainstem damage - there must be an absent pupillary light reaction and the loss of oculovestibular and corneal reflexes
3. lower brainstem damage - indicated by complete apnea
The diagnosis of brain death should ideally be done by more than one senior physician, preferably those not primarily involved in the patient's care.
The principal reason to diagnose brain death is so that the patient's organs can be used for organ transplantation. It is largely accepted nowadays that a patient who has been pronounced as brain dead can have his or her respirator disconnected. Problems usually arise in this respect because of inadequate explanation to or preparation of the family by the physician.
Source - Harrison's Online 2003 - Chapter 24, Acute confusional states and coma.