Generally speaking, an alternate term for bit map. Raster refers to images or type that has been input either by scanning, keyboarding, or other means, processed, and/or output line-by-line. Raster images exist as discrete bytes, pixels or lines of pixels, rather than as vectors, or mathematically described lines and curves. See Bit Map.
In television and video, the term raster refers to the pattern of horizontal scan lines traced by an electron gun on the back of the television screen. A typical raster display (in television equipment) has 525 scan lines per screen (in NTSC; in PAL and SECAM, there are 625 scan lines). A computer monitor works on the same principle, and is also known as a raster display. A computer monitor, however, is simpler in design and construction (as it does not need to decode a broadcast signal), but has a greater number of scan lines, which are in turn subdivided into smaller pixels. A computer monitor, as of this writing, can have over 1,000 scan lines per screen, which includes over one million addressable points.