Death (maraṇa) was traditionally thought to take place when both the heartbeat and respiration ceased but new medical procedures have required a revision of this definition.
Now the medical profession defines death as the point at which life ceases and can no longer be re-established.
The Buddha said death has taken place when vitality (āyu), heat (usmā) and consciousness (viññāṇa) leave the body and it becomes inanimate (acetanā, M.I,296).
Death is one of the most significant events in human experience.
The death of someone close to us usually causes sorrow and grief and the thought of one’s own death can make us alarmed and anxious.
The fear of death probably led to the belief in a realm of eternal life.
If we occasionally contemplate the inevitability of our own death, this can prepare us for it when it finally comes and also make us more appreciative of our life now.
For this reason the Contemplation of Death (maraṇānussati) is one of the contemplations taught by the Buddha (A.III,71).
See; Old Age.
Buddhism and Death, M. Walshe, 1978.