Death is certainly a topic that has interested us, probably since the earliest humans who had evolved the intelligence and language to ponder such things walked the Earth.
Unfortunately, nobody has returned from that state, at least beyond any reasonable doubt currently acceptable to mainstream medical science, to tell the still living what death and post-death is all about.
Of course there are lots on anecdotal tales of death and post-death (the afterlife) - religious texts (hundreds of them), tales of ghosts and ghostly hauntings, spirits communicating from beyond the grave, so-called near death experiences (where you get a preview of things to come), perceptions of previous lives (reincarnation) etc.
Alas, when crunch comes to crunch, anecdotal tales remain just that, anecdotal tales.
Speaking just for myself, I personally can say that (to date) I've never had or experienced any vision or visitation or communication from anyone who has passed away into that great unknown beyond, be they pets or parents, friends or foes.
If there is an afterlife and its 'easy' to crossover and manifest your 'spirit' back in our dimension, you'd think there would be indisputable evidence for that by now.
I mean, you'd think family; loved ones, who have passed on prior to you would, if they could, manifest their spirit selves to you to reassure you about life-after-death, the spirit world, how you get three meals a day, and how everything and everybody is lovey-dovey.
So, either there's no afterlife, or, it's not easy to accomplish a spiritual return to say "hi" to previous loved ones.
One thing is certain, it's a universal fate, something that we will share in common, not only with other humans but with, at least, all other multi-cellular life forms, from mammals to reptiles, amphibians to fish, invertebrates, and much of the plant kingdom as well.
You will boldly go where nearly every living thing prior to you has gone before.
I say 'nearly everything' in that unicellular critters that reproduce asexually achieve a sort of immortality.
In the sense that you, as a multi-cellular critter die, you ain't unique.
You're not being discriminated against! What perhaps makes humans (collectively) unique is that we alone (probably) have a before-the-fact awareness of our demise.
I have to admit I have often wondered whether or not any of my companion animals (pets) and by extension the rest of the animal kingdom (at least), have any perception of their death? Alas, I don't have the ability to ask, and they don't have the ability to answer, that question.
If they don't, is that perhaps a blessing in disguise? Death is, at least from an ecological and biological point of view, essential.
If multi-cellular living organisms were immortal, what point evolution? Evolution wouldn't happen, indeed couldn't happen.
And how would Nature recycle the stuffs essential to future life if said stuffs remained locked up eternally in currently existing life forms? Eventually all essential life-stuffs would be locked away in existing living tissues and no further life forms could be created as there wouldn't exist any more of the 'right stuff' to help them on their way! So, what exactly is death? There's obviously a legal and a medical definition, but it's probably a tad more complicated than the legal and medical niceties make out.
I mean when you (legally and medically) die, the entirety of you, in most circumstances, isn't dead - yet.
One minute after your last breath, most of the individual cells that comprise your body are still very much alive (although it's apparently a myth that your hair and fingernails/toenails will continue to grow for a while - post death).
Leading up to your death, from a whole body point of view, slowly but surely various cells in your body must be dying - they cease to function - until some sort of critical number is reached.
When that number is reached, you collectively die, even though at that time lots of your cells are still left alive and functional, including, I surmise, some of your brain cells, which I also surmise are the critical ones.
Even when pronounced medically dead, at least some of your brain cells are still alive and viable.
Your brain just doesn't die on mass as a lump sum in the space of a few seconds.
One saving grace is that in nearly all cases, death results because of oxygen starvation to the brain.
Anything which stops the transport of oxygen to the brain will cause death.
Such oxygen stoppages include massive bleeding, the heart stopping, your breathing ceasing, choking, an arterial blood clot, fluid in the lungs, etc.
Ultimately, something essential is going to fail inhibiting or preventing oxygen from reaching the brain.
The state of play as that happens is that you lose consciousness prior to your medical and legal death - in other words, you go to sleep first, then die, or in other words, you die in your sleep.
Those final few seconds are trauma free, no matter how traumatic the events causing said oxygen starvation to the brain may have been.
In that sense, death is perhaps somewhat akin to something you've experienced many times - the act of going to sleep, only in this case, you don't awaken again.
That's not so bad, is it? I have observed in my time the final moments of various birds, cats, fish, etc.
- the universal I observed is that their final last moments had every appearance of being peaceful.
I'll grant that there is a difference between losing consciousness while routinely going to sleep - and gaining same as one slowly wakes up - and losing consciousness just prior to dying.
There's the fact that in the former you expect to regain consciousness; in the latter you might well be aware that you won't.
One commonality I suspect is that in both cases you do feel extremely tired, and, as you welcome sleep when seriously fatigued, so to might you welcome 'sleep', however final, just because you are exhausted - for whatever reason (loss of blood or just pure old age).
In any event, events leading to sleep seem to be the closest analog we have to events leading to death.
[Sort of reminds me of the student's prayer - "Now I lay me down to sleep; I pray the Lord my soul to keep; but if I should die before I wake; that's one less exam I'll have to take!] As an aside, I suspect children are far more clued about death than we adults would give them credit for.
Even young kids see death in the form of leaves turning color and falling off trees and eventually rotting; road-kill; mice killed in a trap; dead fished washed up on a beach; squashed insects; the demise of pets (theirs or neighbors); perhaps the death of a neighbor or their friend's parent, etc.
That's quite apart from simulated 'death' experienced via watching TV shows or hearing about real death via the news.
Death is, no matter what your age, something academic (even if still somewhat traumatic), since it isn't yours.
It's only up close and personal when it's yours! By the way, I reject that entire visual "life draining out of the victim's open eyes" concept.
I've seen the open eyes of cats, fish, & birds immediately pre-death and immediately post-death, and I'm damned if I could see any difference.
A negative, I suppose, is that when you die, you die alone.
Even if 100 friends and relatives surround your deathbed, it's YOU facing death, not they.
Even if you are one of 100 passengers in an airplane about to fatally crash - with no survivors - you still face your THE END all by yourself.
Perhaps a case where misery doesn't love or need any company! Anyway, back to the question as what is or results in death, I offer the following which is pure speculation on my part and probably shouldn't be taken seriously - probably.
My basic premise is that death is the result of the irrevocable lose of "The You" - "The You" being that part of the physical you that defines or is your personality, wisdom, emotions, memory, 'soul' (if you will), sense of self, and all other traits that make you an individual; the individual that you are.
That's the loss of your mind in other words.
I suspect that a clue to death is to be found in those unfortunate individuals who do survive after having suffered a prolonged lack of oxygen to the brain.
These 'living vegetables' (as it were), while maybe having perfectly functioning body organs or systems, have lost most of their mind - the inner "The You" - facets, like memory, personality, knowledge, etc.
and as such, usually hence require round the clock, 24/7, care for the rest of their natural lives as the ability to somehow get those essentials back is apparently lost.
The hardware is still there, but the software has been largely deleted or at least severely corrupted.
Notice I said immediately above that most of "The You" is gone in these surviving individuals, but not all.
Death is the total inability of your brain to further store, process, or add to those "The You" elements.
I surmise that lack of oxygen causes those bio-molecules (in the brain) that are part and parcel of housing and processing the inner you -"The You" - to break down, presumably into much simpler bio-chemicals which don't have the ability to store and process those "The You" elements.
Therefore, you lose your abilities and processes essential to such operations, and the longer oxygen starvation goes on, the greater the loss, until "The You" is totally lost, and you die.
Now while that's pretty simplistic, and there's probably much more to the death process I'm sure, it's also pretty probable that lose of "The You" is an essential part of the death process and what death is.
Why? It's clear that you have an automatic nervous system that functions independently of your conscious mind - you breathe in your sleep; you don't have to think about keeping your heart beating, and lots of other body processes are on automatic pilot.
But, "TheYou" has a lot of mastery over your physical body too.
"The You" (call it the conscious mind) can dictate to your body to walk across the room, or do unnatural acts like very rapidly blink your eyelids for no apparent reason - just because you want to.
If 100% of "The You" is lost, the physical body has lost too much of the software (including ultimately the automatic piloting) that's in charge of regulating or controlling it, and thus the physical body, albeit there's a lag time or delay, follows the death process that's already been legally and medically verified.
Perhaps all this is an example of 'mind over matter' - lose the mind, and the matter goes to pot! [Or, if there's no mind, then it doesn't matter anymore!] All of this may be just a very long-winded way of saying that with lack of oxygen, higher brain function areas are the first to feel the effects and the first to go.
And while your physical body could, in theory, be kept on artificial life support for years, what's the point if there is no longer any of "The You" left in that physical body? All of the above equally applies to other 'higher multi-cellular' animals that also have a "The You" component in them.
(Since plants don't have minds, or a "The You" aspect to them, I exclude them from this rambling.
) Anyone who has ever had pets or observed wild animals close up knows they, as humans, are individually unique - with all the traits that humans have, even though they be to a lesser degree.
I don't expect my cats to learn and do calculus though every cat I've owned has been unique in his/her own way(s)! Apart from organic death, there are two other types of death I can think of, but both invoke you not having the reality you think you have.
Firstly, if you are just a figment of someone's dream (or imagination), and they wake up! Secondly, you are part and parcel of someone's computer software, say as in a video game, and then that someone hits the delete key, or kills you off (if its one of those types of video games), exits the program or turns the PC off.
Should the physical you be an illusion, that is, you're actually a virtual person, a product of computer software (never mind who's computer), then concepts central to an afterlife, reincarnation and/or immortality are just other software routines that you can be routed towards at the programmer's whim when the software program that runs the physical you and your environment terminates! The saving grace is that in either case (wetware dreams or software video games) there's the possibility of a resurrection! If death is final, no afterlife, no reincarnation, no prospect of immortality before you're a goner, then the big regret, at least in my case, is never having been able to find out an answer, or the answer(s), to all those questions that's been bugging me for all (or at least most) of my life! Phooey!