The Motive Internet Glossary
dithering
Dithering is approximating a flat colour when only a limited colour palette is available. If the chosen colour is not included in the palette, a pattern of two or more similar colours may be substituted. A brower will dither non-websafe colours when viewing a site on a 256-colour (8-bit) monitor.
Each browser has its own approach to dithering. There can also be discrepancies between the way a browser will dither a foreground and background image.
anti-aliasing, websafe colours
Glossary A-Z