What is meant by "instant death"?

For example, someone was talking about a car accident and said that the driver's
neck was broken, killing him instantly.  What exactly does that mean?

Does a broken neck instantaneously cause the heart to stop beating
and the brain to cease functioning ? If so, why?  

Please do not limit your answer to broken necks.  Regardless of cause,
I'm interested in the speed of death, so just exactly how 'instant' is
'instant death'?  (To me, dead means 'brain dead'.)