In terms of the measurable, definable, world, and the 'they', 'death' is interpreted as that which happens at the end of 'life'. Another word we use for the end of life is 'demise'.
For the purpose of this conversation I will use 'death' (demise) when I am speaking about what happens in the measurable, definable, world and I will use 'death' (Be-ing) when I am speaking of Be-ing.
Be aware, when people speak of 'death' they are most likely speaking about 'demise'. You can't count on them to make the distinction for you. Humans Be-ing readily interchange (confuse) the two words when they come face-to-face with their own mortality and are meaning 'demise' (what happens to a physical body, plant, animal, or man).
Let's clear up the matter of 'demise' first and be done with it. It is rather simple. 'Birth' and 'death' (demise) are two sides of the same coin. When you are 'born', 'death' (demise) is inevitable and nobody can take it away from you or do it for you. It is the only certain/uncertain certainty you have. Just about everything you do between 'birth' and 'death' (demise) is an avoidance of 'death' (demise).
Let me be very clear here. 'Death' (demise) is to be avoided at all cost.
Monuments (headstones, statues, buildings) are an attempt to extend 'living' past the point of 'death' (demise) or immortality. All of this points to one thing and that is: The moment you are born you are already 'dead' (demise), you just don't know when it will happen. Put down the turd, 'death' (demise) and refuse to play patty-cake with it, you can't do anything about it anyway.
Have you ever had a bad enough accident that it made you confront how you are living your life? Have you ever been in a precarious situation and said something along the lines of “If you'll get me out of this I'll never do such and such again” and experienced a shift in your outlook on life? What I just said is a hint that points to 'death' (Be-ing). When you 'close the door' on the way you have been Be-ing, that's another hint that points to 'death' (Be-ing). Those 'hints' are all evidence you need to assure you of the possibility of 'death' (Be-ing).
You should avoid 'death' (demise) at any cost, however, you should run towards 'death' (Be-ing) and experience 'death' (Be-ing) as many times as you can. That's where you'll find 'living' (Be-ing who you are).
'Death' (demise) is a concept we 'play patty-cake' with to remind us to 'live'. However, 'living' is not the opposite of 'death' (demise). 'Living' is something else. It doesn't happen in the realm of the measurable, definable, world, it happens in Be-ing/knowing.
'Death' (demise) is just one of a whole world of concepts we use to hide behind. Your lot in life is to deconstruct the concept of 'death' (demise) and uncover the possibility of 'death' (Be-ing/liv-ing).
Let me say it again. Your lot in life is to de-construct the 'concepts' of life and uncover the possibility that the 'concepts' of life (the measurable, definable, world, and the 'they') don't define who you are. They can only define you as a measurable, definable, thing.
Deconstructing concepts of things like gun, car, airplane are so easy that you don't even take notice. The difficult concepts are the ones that humans Be-ing use to define Be-ing. Since “you should avoid 'death' (demise) at any cost”, when you come close to 'death' (Be-ing) you turn the possibility of not being able “to prove your existence in this world” into a concept to represent Be-ing.
As you de-construct the 'concepts', one by one, and disentangle your 'self' from the labyrinth of measurabilty and definability, you come to a point where you recognize that using the 'measurabilty and definability of the world' to prove your existence never 'captured' who 'you' really are. When you discover that the 'world's concepts can no longer capture 'you', you come face-to-face with the possibility that 'you' can't prove 'you' exist, not even to your 'self'.
Physics has proven that two things cannot occupy the same space, so, when 'you' existing, come face-to-face with the possibility that you don't exist, a very interesting thing happens. When both 'you existing' and 'the possibility that you don't exist' try to occupy the same space, they cancel each other out and both disappear. What gets left in their place is 'you', Be-ing.
What you have just experienced is 'death' (Be-ing).
When you experience 'death' (Be-ing), you will discover that you are no longer a slave to proving/not proving your existence and that now you have 'room' for you to replace 'proving/not proving your existence' with something else. This is the essence of human freedom.
In 'death' (Be-ing) you answer the question "Who am I?"
This is the 'story' (representation) of what happens in "Being and Time" by Martin Heidegger.
Heidegger speaks to Be-ing. It is why I read "Being and Time" 74 times. I was running toward 'death' (Be-ing).