The Opening ends when you have ocmpleted yoour opening principles:
Control the center.
Develop minor pieces toward the center.
Castle.
Connect your rooks.
The middlegame ends when:
Most of the major, and or minor pieces are traded off.
Of course this is all subjective.
There's usually a bit of a blur between when the opening ends and the middlegame begins and when the middlegame ends and the endgame begins.
The middlegame should be the point where you are developed enough that you begin to execute a long-term plan. Unfortunately, a lot of players don't plan much at all, so maybe it's better to say the middlegame begins when you begin doing more tactics and strategy than developing.
The endgame begins when there are relatively few pieces on the board and the strategy is to execute a checkmate or gain in material that then leads to checkmate or push pawns to become queens
So I'm aware that chess is often divided in the opening, the middle game and the end game.
I kind of get the "sense" of what are they and when do they happen but I'd like to know if there is a strict definition of each of them or are they an ambiguous phase of the game?