Brain death is the complete and irreversible loss of brain function (including involuntary activity necessary to sustain life). Brain death is one of the two ways of determination of death, according to the Uniform Determination of Death Act of the United States (the other way of determining death being "irreversible cessation of circulatory and respiratory functions"). It differs from persistent vegetative state, in which some autonomic functions remain. The Australia and New Zealand Intensive Care Society (ANZICS) states that the "determination of brain death requires that there is unresponsive coma, the absence of brain-stem reflexes and the absence of respiratory centre function, in the clinical setting in which these findings are irreversible. In particular, there must be definite clinical or neuro-imaging evidence of acute brain pathology (e.g. traumatic brain injury, intracranial haemorrhage, hypoxic encephalopathy) consistent with the irreversible loss of neurological function." Brain death is used as an indicator of legal death in many jurisdictions, but it is defined inconsistently. Various parts of the brain may keep living when others die, and the term "brain death" has been used to refer to various combinations. For example, although a major medical dictionary says that "brain death" is synonymous with "cerebral death" (death of the cerebrum), the US National Library of Medicine Medical Subject Headings (MeSH) system defines brain death as including the brainstem. The distinctions can be important because, for example, in someone with a dead cerebrum but a living brainstem, the heartbeat and ventilation can continue unaided, whereas in whole-brain death (which includes brain stem death), only life support equipment would keep those functions going. Patients classified as brain-dead can have their organs surgically removed for organ donation; though not everyone agrees with this practice, preferring to limit organ donation to those individuals who have suffered the death of all of their brain and the death of their cardiac and respiratory systems (biological, or full, death). However, if one limits the criteria to those individuals, procuring viable organs can become much more difficult.

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