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  2. Screensaver - Wikipedia

    en.wikipedia.org/wiki/Screensaver

    Before the advent of LCD screens, most computer screens were based on cathode-ray tubes (CRTs). When the same image is displayed on a CRT screen for long periods, the properties of the exposed areas of the phosphor coating on the inside of the screen gradually and permanently change, eventually leading to a darkened shadow or "ghost" image on the screen, called a screen burn-in.