Philosophical Questions Concerning the Meaning of Life and Death
The concept of death is necessarily linked to an irreversible physical change in the state of the organism as a whole. There is a process of dying and there is a process of disintegration, and death is the event which indicates the moment when the process of dying ceases and the process of disintegration begins. This moment is the moment when the brain as a whole ceases to function, when the whole brain has become irreversibly dysfunctional.
Death, so defined as the irreversible loss of function of the organism as a whole, is a singular concept. There are a number of states preceding death where it could be said that personal identity has been lost. A patient on a respirator, in a coma is still alive. Loss of consciousness does not necessarily involve strictly biological concepts, but death does. For ultimately the concept of death can only be applied to organism, not persons. A person in a persistent vegetative state is just that: alive in the most basic biological sense.