Death is the inability of the self to derive sustenance from air. If man can conquer this simple biological deficiency, which has conquered so many of our number, mortal immortality will be his. Which is admittedly a gruesome thought.
Some smart-arse may attempt to counter that many deaths are caused directly by processes other than the inability to derive sustenance from air, but it is clear that were they still deriving sustenance from air, then they would be still in the land of the physically living. One may admittedly be unable to derive sustenance from air for a shortened duration, such as when immersed in a liquid, but be still in a state of life, but were this state to extend beyond a very limited period, then the consquence will be mortal cessation.
Many humans have pondered the nature of life and death, each being apparently the absence of the other, but my definition is assuredly an improvement on all previous efforts in this noble art-form: that life in the physical encasement of the body is the ability of the self to derive sustenance from air, while death is the inability of same. Here kinetic potential is inextricably married to actual application. None that is capable of life is dead, while none incapable of life is alive.