Death is a term that can refer to either the termination of life in a living system, or the state of that organism after that event.
Biologically, death can occur to wholes, to parts of wholes, or to both. For example, it is possible for individual cellss and even organss to die, and yet for the organism as a whole to continue to live; many individual cells can live for only a short time, and so most of an organism's cells are continually dying and being replaced by new ones.
Conversely it is also possible for the organism to die and for cells and organs to live and to be used for transplantation. In the latter case, though, the still-living tissues must be removed and transplanted quickly or they too will soon die without the support of their host.
Irreversibility is often cited as a key feature of death. Accordingly by definition it would not be possible to bring an organism back to life; if an organism lives, this implies that it has not died earlier, even if that seemed the case. Nonetheless, many people do not believe that death is always and necessarily irreversible; thus some have a religious belief in bodily or spiritual resurrection, while others have hope for the eventual prospects of cryonics or other technological means of reversing death.
Biologists believe that the function of death is primarily to permit the operation of evolution.
Historically, attempts to define the exact moment of death have been problematic. Death was once defined as the cessation of heartbeat (cardiac arrest) and of breathing, for example, but the development of CPR and early defibrillation posed a challenge: either the definition of death was incorrect, or techniques had been discovered that really allowed one to reverse death (because, in some cases, breathing and heartbeat can be restarted). Generally, the first option was chosen. (Today this definition of death is known as "clinical death".)
Today, where a definition of the moment of death is required, we usually turn to "brain death" or "biological death": people are considered dead when the electrical activity in their brain ceases. It is presumed that a stoppage of electrical activity indicates the end of consciousness. However, those maintaining that only the neo-cortex of the brain is necessary for consciousness sometimes argue that only electrical activity there should be considered when defining death. In most places the more conservative definition of death (cessation of electrical activity in the whole brain, as opposed to just in the neo-cortex) has been adopted (for example the Uniform Definition of Death Act in the United States).
Even in these cases, the determination of death can be difficult. EEGs can detect spurious electrical impulses when none exists, while there have been cases in which electrical activity in a living brain has been too low for EEGs to detect. Because of this, hospitals often have elaborate protocols for determining death involving EEGs at widely separated intervals.
Because of the difficulties in determining death, under most emergency protocols, a first responder is not authorized to pronounce a patient dead, and if there is any possibility of life and in the absence of a do not resuscitate order, emergency workers must begin rescue and not end it until a patient has been brought to a hospital to be examined by a physician. This frequently leads to situation of a patient being pronounced dead on arrival.
It might also be worthwhile to entertain the possibility that death does not occur at a particular moment, but unfolds as a process over a period of time. Perhaps, in the end, it is not terribly meaningful to speak of "the exact moment of death".
The deceased person is usually either cremated or deposited in a tomb, often a hole in the earth, called a grave. This happens during or after a funeral ceremony. Many other funeral customs exist in different cultures.
Graves are usually grouped together in a plot of land called a "cemetery" or a "graveyard" and are often arranged by a funeral home or undertaker.