A person is legally dead when a licensed physician, having carefully
examined the person, declares that death has occurred.

Before scientists learned that the brain could die before the heart
stops, the only way of determining death was to observe the
absence of heartbeat and breathing. Now we have learned that
once the brain dies, the person is dead and cannot recover.

Brain death is death that results from the total, irreversible loss of all
brain function, due to a severe injury, illness, or disease. A person's
heart will still beat for a period of time after they become brain dead.
It is this relatively rare kind of death that makes organ donation
possible. However, even when a machine (a ventilator) is breathing
for the person, the time between brain death and the heart's stopping
generally does not exceed 24 to 48 hours, so it is important for
decisions to be made about organ donation before the heart stops
beating.