Vertical blank (VBlank)
The pause as the beam finishes the last visible line and travels back to the top — the only safe window to write graphics.
On CRT hardware the image is drawn line by line; when the last line is done the beam is blanked and returned to the top-left corner. During the vertical blank no beam touches the screen, so it is the one moment you can update video memory, scroll and swap sprites without tearing the picture apart.
"Wait for VBlank" is the heartbeat of NES and Game Boy programming: all motion happens in that gap. The idea lives on in VSync and double buffering, which still time the swap to the same interval.