Death is the end of life ; the irreversible cessation of all biological functions that sustain a living organism. Some organisms, such as Turritopsis dohrnii , are biologically immortal ; however, they can still die from means other than aging.
As of the early 21st century, 56 million people die per year. The most common reason is aging , [ 7 ] followed by cardiovascular disease , which is a disease that affects the heart or blood vessels.
Determining when a person 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. Some people believe that all brain functions must cease. Others believe that even if the brainstem is still alive, the personality and identity are irretrievably lost, so therefore, the person should be considered entirely dead.
There are many causes, from accidents to diseases. 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.
The concept of death is the key to human understanding of the phenomenon. Additionally, the advent of life-sustaining therapy and the numerous criteria for defining death from both a medical and legal standpoint have made it difficult to create a single unifying definition.
