Screensaver

A screensaver (or screen saver) is a computer program that blanks the screen or fills it with moving images or patterns when the computer is not in use. Initially designed to prevent phosphor burn-in on CRT and plasma computer monitors (hence the name), screensavers are now used primarily for entertainment, security or to display system status information.

Decades before the first computers using this technology were invented, Robert A. Heinlein gave an example of how they might be used in his novel Stranger In A Strange Land (1961):

Opposite his chair was a stereovision tank disguised as an aquarium; he switched it on, guppies and tetras gave way to the face of the well-known Winchell Augustus Greaves.

Many modern television operating systems, media players and other digital entertainment systems include optional screensavers.

Some screensavers also offer additional functions like automated workstation password protection during inactivity (i.e., probably absent user) or disk scan with installed antivirus software.

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 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. Cathode ray televisions, oscilloscopes and other devices that use CRTs are all susceptible to phosphor burn-in, as are plasma displays to some extent.

Screen-saver programs were designed to help avoid these effects by automatically changing the images on the screen during periods of user inactivity.

...
Wikipedia