A heliodon (HEE-leo-don) is a device for adjusting the angle between a flat surface and a beam of light to match the angle between a horizontal plane at a specific latitude and the solar beam. Heliodons are used primarily by architects and students of architecture. By placing a model building on the heliodon’s flat surface and making adjustments to the light/surface angle, the investigator can see how the building would look in the three dimensional solar beam at various dates and times of day.
It's a Greek-to-English neologism, so I don't think it quite belongs as a Cool Word of The Month (although I once chose panmictic), but I just think it's a nifty invention. We have one in our College of Architecture that is used for various visualizations. I think darana and phawkwood have seen it; anybody else who's reading this?