screen saver

noun Computers.
  1. a program that displays a constantly shifting pattern on a screen when the computer is idle, originally used to prevent damage to the screen through continuous display of the same image but now used primarily for decoration.
Also screen·sav·er [skreen-sey-ver] /ˈskrinˌseɪ vər/
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

British Dictionary definitions for screensaver

screensaver

noun
  1. a computer program that reduces screen damage resulting from an unchanging display when a computer is switched on but not in use by blanking the screen or generating moving patterns, etc
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012