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, and brain death is sometimes used as a legal definition of death. The remains of a former organism normally begin 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 other means than aging.

Figuring out when someone is dead has been a problem. Initially, there was the definition of death when breathing and the heartbeat ceased. However, the spread of CPR no longer meant it was irreversible. Brain death was the next option, which fractured between different definitions. Some people believe that all brain functions must cease. Some believe that even if the brainstem is still alive, their personality and identity are dead, so therefore, they 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. 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 the idea of an afterlife and also may hold the idea of judgment of good and bad deeds in one's life. There may also be different customs for honoring the body, such as a funeral, cremation, or sky burial.

Death is actively trying to be cured by a group of scientists known as biogerontologists, through seeking to do the same as biologically immortal organisms do and applying a similar means to humans. 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.