Sentence:  Planes farther back on the z-axis scroll more slowly than those in front of them, producing a <WSD>parallax</WSD> effect. Early games with parallax scrolling tended to have their action taking place all in the forefront plane ... 
Meaning of the word marked with <WSD> tags:  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.