Merriam-Webster Collegiate Dictionary - raster

 noun  Etymology: German, from Latin ~, rastrum rake, from radere to scrape  Date: 1934 a scan pattern (as of the electron beam in a cathode-ray tube) in which an area is scanned from side to side in lines from top to bottom; also a pattern of closely spaced rows of dots that form an image (as on the cathode-ray tube of a television or computer display)