What is Death? What did Buddha say regarding the experience of Death? What is death according to Buddhism? When does it occur? Why do so many people die simultaneously in natural calamities? What did Buddha say about death? Please give references of Buddha's teachings.
What is death acoording to Buddhism? Here is one definition

http://www.urbandharma.org/udharma5/viewdeath.html


  Death and the impermanence of life
  
  In the teaching of the Buddha, all of us will pass away eventually as
  a part in the natural process of birth, old-age and death and that we
  should always keep in mind the impermanence of life.  The life that we
  all cherish and wish to hold on. 
  
  To Buddhism, however, death is not the end of life, it is merely the
  end of the body we inhabit in this life, but our spirit will still
  remain and seek out through the need of attachment, attachment to a
  new body and new life. Where they will be born is a result of the past
  and the accumulation of positive and negative action, and the
  resultant karma (cause and effect) is a result of ones past actions.
  
  This would lead to the person to be reborn in one of 6 realms which
  are; heaven, human beings, Asura, hungry ghost, animal and hell. 
  Realms, according to the severity of ones karmic actions, Buddhists
  believe however, none of these places are permanent and one does not
  remain in any place indefinitely. So we can say that in Buddhism, life
  does not end, merely goes on in other forms that are the result of
  accumulated karma. Buddhism is a belief that emphasizes the
  impermanence of lives, including all those beyond the present life. 
  With this in mind we should not fear death as it will lead to rebirth.
  
  The question about when death occurs is even more important, because to a practitioner of Buddhism, death is not an eventuality that will arrive someday. Death and meeting death is part of our daily practice of meditation. We don't think of the body dying but day by day we face and face again the temporal nature of life and ask ourselves if we are making the most of life for spiritual advancement by learning to come to terms with death.


Here is an illustrative story from a PBS special on Buddhism
http://www.pbs.org/wgbh/pages/frontline/shows/tibet/understand/dying.html


  An example of this attitude can be found in the bioraphy of Milarepa,
  who began his meditative practice after having killed a number of
  people through black magic. The realization of his impending death and
  the sufferings he would experience in his next lifetime prompted him
  to find a lama who could show him a way to avert his fate. His concern
  with death was so great that when he was medititing in a cave his
  tattered clothes fell apart, but he decided not to mend them, saying,
  "If I were to die this evening, it would be wiser to meditate than to
  do this useless sewing."1
  
  This attitude epitomizes the ideal for a Buddhist practitioner,
  according to many teachers. Atisha is said to have told his students
  that for a person who is unaware of death, meditation has little
  power, but a person who is mindful of death and impermanence
  progresses steadily and makes the most of every precious moment. A
  famous saying of the school he founded, the Kadampa, holds that if one
  does not meditate on death in the morning, the whole morning is
  wasted, if one does not meditate on death at noon, the afternoon is
  wasted, and if one does not meditate on death at night, the evening is
  wasted.


Here are a couple quotes attributed to the Buddha that imply what Buddha thought most important, cessation of suffering and Nibbana.
http://www.life-changing-inspirational-quotes.com/buddha-quotes.html#Death


  Buddha Quotes on Death
  
  "Even death is not to be feared by one who has lived wisely."
  
  "It is better to travel well than to arrive."


This trend of thinking beyond life and death continues here
http://en.wikiquote.org/wiki/Gautama_Buddha 


  Even death is not to be feared by one who has lived wisely. As quoted in Wisdom for the Soul: Five Millennia of Prescriptions for Spiritual Healing (2006) edited by Larry Chang, p. 193 This is actually a pithy modern-day 'summary' of the "Abhaya Sutta" (AN 4.184). It appears in "Buddha’s Little Instruction Book" by Jack Kornfield (p88).


Another example that Shakyamuni Buddha reveals his lack of concern about death is under the Bodhi tree before his realization.
http://en.wikiquote.org/wiki/Gautama_Buddha 


  Let my skin and sinews and bones dry up, together with all the flesh
  and blood of my body! I welcome it! But I will not move from this spot
  until I have attained the supreme and final wisdom. The Jatka (From
  the Attainment of the Buddhaship. Also is in the Nirvana Sutta.)


The essence of the Buddha's teaching is not to cling to physical existence and in the same line do not fear death. What dies is the life we conceptualize in our mind. What is untouched by death is the consciousness that the Buddha invites us to develop in our practice.