What is the medical definition of death?
Ok. So when a doctor declares a person dead, it is when their heart or brain is totally not working, and won't start working no matter what they do.

This is legal death.

If your heart stops cold, you are clinically dead. This is reversible, if you are lucky. This definition basically only exists so people can say they "died". It has no real biological significance, except for the fact that your heart is not pumping blood and providing you with oxygen. This will cause...

Biological death:
Within 5-10 minutes at room temp you have reached the event horizon of biological death. Biological death is real death.

Once this time threshold is crossed, there is almost always no going back.
This is because Cells in the brain are destroyed by oncosis, and other mechanisms and forces far beyond me.