Scanout
The process by which the display controller reads the framebuffer row by row and streams it to the monitor at its refresh rate.
Once a frame sits in the framebuffer it isn't finished until it has been shown, and that job falls to a separate circuit — the display controller, historically the CRTC. It reads the memory at a relentless cadence, line by line from top to bottom, clocking the pixels out to the screen 60 or 144 times a second. This scanout proceeds whether or not the GPU has managed to draw anything new.
The whole family of display phenomena bottoms out here. Swap the framebuffer mid-scanout and the boundary shows as screen tearing; wait for the gap between two reads — the vertical blank — and you get clean vsync. Double buffering exists precisely to give the GPU one buffer to draw into while the controller scans out the other.