An irreversible process of deterioration in the body's systems and organs. The breakdown of these processes, the sum of which is life, causes a loss of organic functions or death.
DeSpelder LA. Strickland AL. 2002. The Last Dance: Encountering Death and Dying. 6th Ed. New York, N.Y.: McGraw Hill. p. 211-2
An Alternative Definition
Normal cellular function involves the production of free energy required for vital cellular metabolism, the production of enzymatic and structural protein, the maintenance of chemical and osmotic homeostasis of cell, and cell reproduction. During normal functions, cells require oxygen, phosphate, calcium (C, H, N, O, P, S), nutritional substrates, ATP, which is required as a source of free energy, intact cell membranes, and a steady-state activity that requires O2 consumption.
Cell death may occur when any of these functions is interrupted.
Cellular Death - http://en.wikipedia.org/wiki/Death