Game loop
Also said as Main loop, Update loop
The circle at the centre of every game: read input, update the world, draw the picture, then do it again, usually dozens of times a second. One turn of it makes one frame.
A normal program waits for you. A game cannot, because the world has to keep moving whether you press anything or not. Everything about a game's timing follows from where in this circle a thing happens.
Say it
“Read input at the start of the loop and apply it in the same frame, so a press never waits a whole extra frame to take effect.”
Where you’ve seen it
id released Doom's source code, and its D_DoomLoop function is the shape in the open: start the tic, process events, run the game, update the sound, display.
Not to be confused with gameplay loop
The gameplay loop is what the player repeats: fight, loot, upgrade. The game loop is what the computer repeats, sixty times a second.