The brain is truly dead when you progress too far in cell death. For example, when you die from drowning, you are dead because oxygen starvation caused too much cell death (and you can survive drowning even if you had some cell death, so long as it was limited).
There have been recent developments in the medical field that suggest that seemingly-dead avalanche victims, whose body temperatures have dropped to freezing or below and whose body functions have all but ceased, can be revived if you warm them... slowly. (Warming them too quickly causes some sort of shock)
There was also an article on HN 6 months ago or something, where a doctor rescued a person in cardiac arrest by keeping her head cold enough using groceries just bought in the supermarket (if I remember correctly) - at about 25-30C. The person was successfully reanimated about 4 hours after the cardiac arrest and suffered no brain damage.
I have seen fist hand someone who had drowned, no pulse, being brought back with CPR. It happened to be ice cold glacial melt kayaking in the Alps that they had drowned in. Not 4 hours, but a significant amount of time without a pulse and breathing. Apparently he was OK afterwards.
There have been recent developments in the medical field that suggest that seemingly-dead avalanche victims, whose body temperatures have dropped to freezing or below and whose body functions have all but ceased, can be revived if you warm them... slowly. (Warming them too quickly causes some sort of shock)