Some patients are declared brain dead and then begin spontaneously breathing hours later. Medical scientists say it doesn't matter because most brain-dead patients do not come back to life, but a rigorous scientist would say that these cases speak loudly about the flaws in our criteria for death. And yes — death to a cardiologist means that your heart has stopped, and he can't get it to restart. But to a neurologist, it might mean something else. In 1968, a committee at Harvard Medical School put forth an article stating that there is a second kind of death: brain death. Even though your heart is still pumping, and you're still able to breathe on a ventilator, if your brain stem is down, you're dead. This theory was made law in all 50 states in 1981, so now in the U.S. we have two kinds of death: real death (cardiopulmonary death) and what some doctors call "pretty dead," or brain death. A cell biologist, on the other hand, may have a standard more rigorous than cardiologists or neurologists. They might want to see all one's cells dead, which we call putrefaction.
He goes on to talk about how our complex criteria for death has evolved over history, and where it may be tomorrow. Link -via Not Exactly Rocket Science