Source : WordNet®

screen saver
     n : (computer science) a moving design that appears on a
         computer screen when there has been no input for a
         specified period of time; "screen savers prevent the
         damage that occurs when the same areas of light and dark
         are displayed too long"

Source : Free On-Line Dictionary of Computing

screen saver
     
         A program which displays either a completely black
        image or a constantly changing image on a computer monitor to
        prevent a stationary image from "burning" into the phosphor of
        the screen.  Screen savers usually start automatically after
        the computer has had no user input for a preset time.  Some
        screen savers come with many different modules, each giving a
        different effect.
     
        Approximately pre-1990, many {cathode ray tubes}, in TVs,
        computer {monitors} or elsewhere, were prone to "burn-in";
        that is, if the same pattern (e.g., the {WordPerfect} status
        line; the {Pong} score readout; or a TV channel-number
        display) were shown at the same position on the screen for
        very long periods of time, the phosphor on the screen would
        "fatigue" and that part of the screen would seem greyed out,
        even when the CRT was off.
     
        Eventually CRTs were developed which were resistant to burn-in
        (and which sometimes went into {sleep} mode after a period of
        inactivity); but in the meantime, solutions were developed:
        home video game systems of the era (e.g., Atari 2600s) would,
        when not being played, change the screen every few seconds, to
        avoid burn-in; and computer screen saver programs were
        developed.
     
        The first screen savers were simple screen blankers - they
        just set the screen to all black, but, in the best case of
        {creeping featurism} ever recorded, these tiny (often under 1K
        long) programs grew without regard to efficiency or even basic
        usefulness.  At first, small, innocuous {display hacks}
        (generally on an almost-black screen) were added.  Later, more
        complex effects appeared, including {animations} (often with
        sound effects!) of arbitrary length and complexity.
     
        Along the way, avoiding repetitive patterns and burn-in was
        completely forgotten and "screen savers" such as {Pointcast}
        were developed, which make no claim to save your monitor, but
        are simply bloated {browsers} for {push media} which
        self-start after the machine has been inactive for a few
        minutes.
     
        (1997-11-23)