Parallax
Also said as Parallax scrolling, Layer scrolling
Background layers move slower than the foreground as the camera moves, the same way that fence posts rush past a train window while hills barely shift. It fakes depth on a flat screen.
It is a cheap, strong depth cue that tells you which things are close. Too much, or layers moving at the wrong speed, makes the world feel like sliding paper or even makes people feel sick.
Say it
“Use four parallax layers: sky fixed, mountains at 10% of camera speed, trees at 40%, foreground bushes at 130%. The play layer is always 100%.”
Where you’ve seen it
Moon Patrol (1982) is often credited with introducing full parallax scrolling, with three background layers moving at different speeds. Jump Bug (1981) had used a limited form. Ori and the Blind Forest pushes the idea to hundreds of layers, by one reviewer's count.
Not to be confused with layers
Layers are the separate pictures. Parallax is only their different speeds. Layers all moving together are just one flat picture.