A screensaver is a programme which, after a pre-set time during which the computer has not been used, replaces the image onscreen and displays moving objects to protect against "screen burn" (damage to the screen caused by a stationary image being displayed for too long and burning the phosphor).

(Collin, S.M.H. [ed.], Dictionary of Multimedia, Peter Collin Publishing, Teddington, Middlesex, 1995, pp232-3)