The area of picture into which it is considered safe to place material, graphics, text or action, so that it will be viewable when received at home. Initially this was necessary with 4:3 aspect ratio CRT screens as they were always overscanned to avoid showing the ‘black’ that surrounded the active picture. Typically 5% in from the edges was considered safe. Today most TV screens are flat panels, and the pictures are 16:9. There are controls for zooming or changing the aspect ratio, but usually all the active picture is displayed on the screen.
See also: Aspect ratio (of pictures)