Death is the irreversible breakdown and failure of a living system. The consequence of death is decay.
There is no clear dividing line between death and life; as medical technology advances states of body breakdown previously considered irreversibly become reversible. There are a number of different standards used to determine if someone is dead or alive. Originally death was considered to be the cessation of heart beat and breathing; however the development of CPR made this reversible in some cases, and today this is refered to as clinical death.
The standard generally used today is brain death, or the irreversible cessation of brain activity. There has been however disputes over whether a complete cessation of brain activity is necessary, or merely a cessation of brain activity in the neo-cortex, which is necessary for consciousness. In most places the more conservative definiton of death has been adopted (e.g. the Uniform Definition of Death Act in the United States).
It is a vexed question what if anything lies after death for human beings.
See also cryonics.