Vector is part of the category Monitor Type.
The most common type of display for arcade games was a raster monitor where the image consisted of a grid of pixels that was generated by an electron beam sweaping across the image line by line. In contrast, a vector monitor (sometimes also referred to as X-Y) created images by having the electron beam draw lines directly between arbitrary points (similar to an oscilloscope). The resulting image avoided some of the common problems exhibited by a raster display, most notably aliasing and pixelation, providing a very clear and sharp display. There were, however, drawbacks such as being unable create an area of solid color limiting the objects on screen to only outlines or the potential for flicker if too many shapes needed to be drawn. While most games with vector monitors were black & white, a few later ones used color.
See also: Raster Monitor