just to agree with somebody above, I was reading the maxwell render manual yesterday and it looks pretty good IMVHO. they are using the terms "reflected caustics" and "refracted caustics". if one looks at carrara renders of diamonds, one also sees those concentrated areas of light that are generated when caustics are enabled. they are likely due to repeated internal reflection and consequent refraction upon exit, which forces some beams to converge, due to the radial symmetry of the diamond cut. some may use the term "brilliance" (rather than caustics) when referring to the physical phenomenon.