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You’ve probably seen a parallax website before. Especially common on long, single-page scrolling sites, images in the background seem to scroll at a different speed than the text or images in the foreground. That’s called parallax.
According to web design site Awwwards:
The parallax effect uses multiple backgrounds which seem to move at different speeds to create a sensation of depth (creating a faux-3D effect) and an interesting browsing experience.
They have some great examples.
Since, as you know, I like to dig into the history of marketing verbiage, parallax comes from the Greek parallaxis, meaning “alteration.” Parallax is a difference in the apparent position of an object viewed along two different lines of sight, and is used in various fields. Astronomers use it to measure distance between stars. Rifle scope need to account for parallax to overcome differences in how a shooter perceives the crosshairs (foreground) and target (background). Video game developers use it to create a sensation of depth in first person games. And more.