Death is the irreversible cessation of all biological functions that sustain an organism. For organisms with a brain, death can also be defined as the irreversible cessation of functioning of the whole brain, including the brainstem. Brain death is sometimes used as a legal definition of death. The remains of a former organism normally begins to decompose shortly after death. Death is an inevitable process that eventually occurs in all organisms. Some organisms, such as Turritopsis dohrnii, are biologically immortal. However, they can still die from means other than aging.
Determining when someone has definitively died has proven difficult. Initially, death was defined as occurring when breathing and the heartbeat ceased, a status still known as clinical death. However, the development of CPR meant it was no longer strictly irreversible. Brain death was the next option, but several definitions exist for this. Some people believe that all brain functions must cease. Some believe that even if the brainstem is still alive, the personality and identity are irretrievably lost, so therefore, the person should be entirely dead.
Death is generally applied to whole organisms; the similar process seen in individual components of an organism, such as cells or tissues, is necrosis. Something that is not considered an organism, such as a virus, can be physically destroyed but is not said to die, as a virus is not considered alive in the first place. As of the early 21st century, 56 million people die per year. The most common reason is cardiovascular disease, which is a disease that affects the heart.
Many cultures and religions have a concept of an afterlife that may hold the idea of judgment of good and bad deeds in one's life. There are also different customs for honoring the body, such as a funeral, cremation, or sky burial.
A study known as biogerontology seeks to eliminate death by natural aging in humans, often through the application of natural processes found in certain organisms. However, as humans do not have the means to apply this to themselves, they have to use other ways to reach the maximum lifespan for a human, such as calorie reduction, dieting, and exercise.
As of 2022, a total of 109 billion humans have died, or roughly 93.8% of all humans to ever live.