Amara, I think your question is more to do with "what is life?" rather than "what is death?"
If we state and understand that death equals the cessation life, then that becomes our constent. If death is the cessation of life, the variable and dependent for the equation is life. What is life?
If we as humans can not comprehend what life is; what the meaning of life is; or what the purpose of life is, than it is not possible for us to begin to comprehend what death is, other than a lack of life.
Therefor when we view death, our idea and explenation is always confound by our human perception and experiance, thus we define death based on our own experiance; death is a lack of experiance; death is when we stop breathing, thinking, and functioning in an accepted and understood, yet limited to human understanding, way.
Therefor our understanding and explenation of death can only be based on our physical and mental understanding, and proof of death can never include spiritual life, or even physically aspects we do not understand or can not experiance. I say physical in regards to energy, as when we die, our life energy does not stop, it simply leaves our body and moves on. This we can not understand and therefor do not factor into our definitions of death. It is interesting to note that the explenation of life energy leaving the body and joining the rest of the energy in the world, has a strong parallel in the belief that a soul leaves the body and joins the rest of the souls.
So, what is death? I dont know, and the reason I do not know, is that I do not know the full spectrum of life, and thus do not know what a true lack of life is.