Brain death is used as a legal indicator of death in many jurisdictions. It is characterized by the complete and irreversible loss of all brain activity. Death was traditionally determined through the end of respiration and heartbeat, yet the abilities of modern medicine to keep people with no heartbeat or respiration alive called for better a definition of death.
However, a rigorous definition of death will always stay complicated and there is an ongoing discussion whether the concept of brain death should play the central role it does at the moment.
Thomas Jagau
Read more: http://www.deathreference.com/Bl-Ce/Brain-Death.html and references therein.