When does "Endgame" begin?

Curious as to which factors determine the game is now in it's "endgame" stage?   

When you realize you have to visualize the entire board in order to make most tactics viable. When your calculating moves based on entire rows, columns and diagonals, or thinking in terms that if you move this or that in the game, then a few moves later, the entire focus of the board moves for whatever position is currently the focus of the board to another wholly different section of the board.

Frequently, there could be simultaneous action on different parts of the board. For example, a rook might be supporting the promotion of a pawn on the far corner of the board, while most of the other few pieces on the board are clustered as far away as possible with your opponent trying to promote a pawn of his own.

There is no clear line, which is also mentioned here: https://en.wikipedia.org/wiki/Chess_endgame

Generally in the middle game, you start thinking in terms of what the endgame might look like. It's very common at some point that part of the board looks like an endgame but a different part of the board would be described as still in the middlegame.

The king is active. That's why there are also queen endgames, the king usually loses some weight there.

I say when there is around 15 points on each side. 

So 2 rooks, bishop and two pawns for each side I would consider endgame

all knights and bishops + 3 pawns also. 

Etc.

When the material available is difficult to execute Checkmate for either side, I guess,  

Nimzovich might have said it was when there were only crumbs left scattered from the feast that was the middle game, except that it wasn't. 

Interesting thoughts... I wonder if you had 100 grandmasters all looking at the same game at one point in the game... how many would say that point is the endgame, and then explain why.  Would be great to see their thought process behind it. 

Very true!

5 days and no witty response to this comment yet.

Terminology doesn't really matter does it? "A rose by any other name." We were supposed to read that in high school right?

Yeah, that's more to the point.

Witty isn't really his area.

He can post some screenshots of his phone-engine though, because that's always relevant right?

But yeah, a popular answer is when the king can safely come out.

And GOP's answer is something of a corollary: when pawn promotion overtakes checkmate as a priority (although that can happen earlier too)