WordNet (r) 3.0 (2006):

screen saver
    n 1: (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"

The Free On-line Dictionary of Computing (30 December 2018):

screen saver
phosphor fatigue
screen blanker

    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)