Death is generally applied to whole organisms; the similar process seen in individual components of a living organism, such as cells or tissues, is necrosis. Something that is not considered a living organism, such as a virus, can be physically destroyed but is not said to die.[from wikipedia]

The concept of death is a key to human understanding of the phenomenon. There are many scientific approaches and various interpretations of the concept.[from wikipedia]

One of the challenges in defining death is in distinguishing it from life. As a point in time, death would seem to refer to the moment at which life ends.[from wikipedia]

It is possible to define life in terms of consciousness. When consciousness ceases, a living organism can be said to have died. One of the flaws in this approach is that there are many organisms that are alive but probably not conscious (for example, single-celled organisms).[from wikipedia]

Historically, attempts to define the exact moment of a human's death have been subjective, or imprecise. Death was once defined as the cessation of heartbeat (cardiac arrest) and of breathing, but the development of CPR and prompt defibrillation have rendered that definition inadequate because breathing and heartbeat can sometimes be restarted. [from wikipedia]

Today, where a definition of the moment of death is required, doctors and coroners usually turn to "brain death" or "biological death" to define a person as being dead; people are considered dead when the electrical activity in their brain ceases.[from wikipedia]