"Death" is the termination of the biological functions that define living organisms. It refers both to a specific event and to a condition, the true nature of which it has for millennia been a central concern of the world's religious traditions and philosophers to penetrate; in particular, the possibility or otherwise of what is known as life after death.

Numerous factors can cause death: predation, disease, habitat destruction, senescence, suicide, conflict, malnutrition, or mere accidents resulting in terminal physical injury. The principal cause of death in people in developed countries is disease precipitated by aging. The chief concern of medical science has been to postpone and avert death. Precise medical definition of death, however, becomes more problematic, paradoxically, as scientific knowledge and technology advance.

Fate of dead organisms

The following description assumes that the death itself has not occurred from being burnt, pulverized, or vaporized...

In animals and humans small movements of the limbs (for example twitching legs or wings) known as a postmortem spasm can sometimes be observed for some time following death. Pallor mortis is a postmortem paleness which accompanies death due to a lack of capillary circulation throughout the body. Algor mortis describes the predictable decline in body temperature until ambient temperature is reached. Within a few hours of death rigor mortis is observed with a chemical change in the muscles, causing the limbs of the corpse to become stiff (Latin rigor) and difficult to move or manipulate. Assuming mild temperatures, full rigor occurs at about 12 hours, eventually subsiding to relaxation at about 36 hours; however, decomposition is not always a slow process. Fire, for example, is the primary mode of decomposition in most grassland ecosystems.

Some organisms have hard parts such as shells or bones which may remain intact after decomposition has occurred. These remains can, over time, become fossils, which are the mineralized or otherwise preserved remains or traces (such as footprints) of animals, plants, and other organisms. Fossils vary in size from microscopic, such as single cells, to gigantic, such as dinosaur fossils. A fossil normally preserves only a small portion of the deceased organism, usually that portion that was partially mineralized during life, such as the bones and teeth of vertebrates, or the chitinous exoskeletons of invertebrates. Preservation of soft tissues, such as in mummification, is extremely rare in the fossil record.

Competition, natural selection and extinction

Death is an important part of the process of natural selection. Organisms that are less adapted to their current environment than others are more likely to die having produced fewer offspring, reducing their contribution to the gene pool of succeeding generations. Weaker genes are thus eventually bred out of a population, leading to processes such as speciation and extinction. It should be noted however that reproduction plays an equally important role in determining survival, for example an organism that dies young but leaves many offspring will have a much greater Darwinian fitness than a long-lived organism which leaves only one.