A television technology term.
Most televisons still use a cathode ray tube to display its pictures. A cathode ray tube works by sweeping its beam across the screen from left to right (and slightly down). This sweep is called a 'raster'
So an engineer might describe a television as having a 'raster scan of 25 cycle per second'. This would mean that the beam is sweeping across the screen 25 times a second. If it was slowed down enough, then you would see a zig-zag of light going down the screen.
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Click on this link: Raster