Source : WordNet®

raster
     n : formation consisting of the set of horizontal lines composed
         of pixels that is used to form an image on a CRT

Source : Free On-Line Dictionary of Computing

raster
     
         The area of a video display that is covered by
        sweeping the electron beam of the display in a series of
        horizontal lines from top to bottom.  The beam then returns to
        the top during the vertical flyback interval.
     
        See also {CRT}, {frame buffer}.
     
        (1995-03-22)