parallax

parallaksi An apparent shift in the position of two stationary objects relative to each other as viewed by an observer, due to a change in observer position.
2008, Bernard Perron, ‎Mark J. P. Wolf, The Video Game Theory Reader 2 (page 157)

Planes farther back on the z-axis scroll more slowly than those in front of them, producing a parallax effect. Early games with parallax scrolling tended to have their action taking place all in the forefront plane (..)