Raster

A raster display is one which uses, or takes as an input signal, a scan across the screen (usually a CRT) consisting of multiple horizontal lines (the rasters), laid down from the top to the bottom of the screen, with a short, constant vertical distance between them.

After scanning across the screen on one raster, the beam must then return to the other edge where it started, during which nothing is shown on the display (it is said to be 'blanked'); this is known as the 'horizontal retrace'. Likewise, when it gets to the bottom of the screen, it must return to the top, which is called the 'vertical retrace'.

This technique for covering the screen was first used in televisions, and was later adopted by video terminals and other video displays. A similar technique is used by laser printers.