Hardware scrolling
Scrolling the background by changing a scroll register in the graphics chip instead of redrawing the whole screen in software.
The graphics chip — a PPU, VDP or the like — reads a tilemap at an offset held in registers. Bumping the register by one shifts the entire background a pixel, for free, without the CPU touching a single dot. New tiles are streamed into the edge that has just come into view, usually just off the visible area, while raster interrupts can split the screen into zones that scroll at different speeds to suggest depth.
This was the technique that made fluid side-scrolling possible on 8- and 16-bit machines like the NES, Mega Drive, C64 and Amiga. Machines without it — early IBM PCs with CGA — essentially could not scroll a game world smoothly, which long kept platformers on the consoles. See smooth scrolling for what the registers were really after.