An Acronym for Cathode Ray Tube.

The main component of traditional monitors and TV screens. Inside the tube there is vacuum, through which a high voltage accelerator in the back fires a stream of electrons at the phosphor coating on inside of the front of the tube. The phosphor particles hit by the electrons momentarily emit light. An electrically induced magnetic field bends the trajectory of the ray in varying directions so that it scans over the entire phosphor surface.

By covering the entire area quickly enough, it can be started over soon enough that the electrons can be fired at previously illuminated phosphor particles as soon as or just before they go completely dark. This way, an illusion of a steady image can be created despite the ray lighting up only tiny areas of the screen at a time.