Low dispersion glass and its variants - ultra-low dispersion (UD) glass, extra-low dispersion (ED) glass - is expensive optical glass which can reduce unwanted colour fringing and other optical problems in lenses, particularly long telephoto lenses.

Dispersion is the rainbow effect seen with prisms and the like - white light being split up into a rainbow spectrum of its constituent wavelengths. Low dispersion glass does not disperse white light as much as regular glass, thereby reducing the amount of correction required to compensate for the phenomenon.

What is fluorite?

Technically, calcium fluorite isn’t glass. It’s actually a type of crystal produced synthetically by Canon and used in many of their top of the line L series lenses in place of low dispersion glass. It’s an expensive material but is extremely useful for minimizing optical lens aberrations, particularly with telephoto lenses.