Once, a while back, I said that death is the cessation of self-regulation of essential autonomic functions AND consciousness/higher mental functions. Given this is more of a cognitive scientists view however, it does not really apply to simple creatures such as prokaryotic microorganism. Or even a dog per se, as the level of consciousness actually present is seemingly so small it is insignificant (Note, I am not using the word "consciousness" in the medical sense or mere perceptual awareness) and past any psychometric means of measurement. In such cases, the organisms cessation of self-regulation of essential autonomic functions is enough to label the organism as "dead". And there you have it, the best definition of death I can give >.< .
As for the subject of an afterlife, while a great deal of speculations have been made in various forms, none of them can carry any particular validity. Take for example, the religious assertion that one's "soul" lives on after death. How can that be proven? The best such beliefs have are written texts and the words of a leader, none of which can be taken with any serious validity as they both could be lies, unfounded opinions, or forgery. Likewise, being unquantifiable and unverifiable, it has no holding in science. Ergo my assertion that no speculation hitherto has been valid. Nor do I see it being so in the near future.