Typically pronounced “abbey” (like where monks live). Also called “V-number”.
This value describes how differently a transparent material will bend different colors of light that pass through it, aka the amount of dispersion. High Abbe numbers represent materials that have low dispersion. Lower Abbe numbers represent higher dispersion, or materials that bend red light a lot differently than than they bend blue light (and all the colors in between).
So, if you want to make a prism that separates white light into the many colors it’s made of, you’re better off creating that prism with a low Abbe number material. Usually, though, high dispersion is not a desirable characteristic when designing an optical system – especially in imaging optics. If Abbe number is an important factor in your system, usually you’d be looking for materials with a high Abbe number, or low dispersion.