3dRender.com : Glossary : Caustics, Caustic Shading

Caustics appear beside a glass

caustics (or caustic shading) - A visual effect seen when light is reflected off a specular or reflective surface, or focused through a refractive surface, so that it indirectly illuminates other surfaces with focused light patterns.  In 3D graphics, caustics are rendered as a type of global illumination, often using Photon Mapping or Bi-directional Raytracing techniques.

In the picture above, you can see how placing a glass in a beam of light reflects caustic patterns to the left and right of the glass, and refracts more caustic patterns through the glass onto the table.

Web Link: A Look At Caustics - An article with more explainations and sample renderings of caustics.

Source: Digital Lighting & Rendering by Jeremy Birn, adapted from the book by permission.  Page and image © 2001 by Jeremy Birn.