WordNet (r) 3.0 (2006):

raster
    n 1: the rectangular formation of parallel scanning lines that
         guide the electron beam on a television screen or a
         computer monitor

The Free On-line Dictionary of Computing (30 December 2018):

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)