Death is the natural state of an organism following life, at which time all life processes (such as breathing, brain activity, etc.) cease.
Many people find the thought of their own death, or that of their loved ones, to be extremely troubling. This might be one reason certain aspects of religion, such as certain burial traditions and belief in an afterlife, began in early humans in the first place.
Biological Versus Legal
Death is currently known as an irreversible reaction in all organisms, however, there are many known cases where people their hearts had stopped beating but were revived by a defibrillator. For this reason, the definition of permanent death in the legal and biological sense does not entirely rest on the activity of the heart. Absolute unrecoverable death entails the irreparable destruction of the brain which can be caused by oxygen deprivation, bleeding of the brain, incineration, or high impact trauma. In the distant future, when the entire physical state of a brain can be copied at the necessary resolution to preserve fidelity to the original, the definition of death may change. The ability of nanotechnology to reconstitute the brain after it is destroyed to the exact state it was before the destruction may also make true death more difficult to establish.
Homeostasis is a phenomenon that is what defines living organisms. If humans or their descendants attain non-biological physical forms that only act as platforms for the continued operation of the mind, then according to modern definitions of life, these transhumans/posthumans would not be considered living since their cells (or the nanotech equivelents) will not necessarily have to exhibit homeostasis in order to facilitate the primary function of simply acting as a platform for consciousness.