a filter used in photography to stop UV light from reaching the film.
Notice that a great deal of the UV light in natural scenes is already stopped by the very optical glass of the lens. UV filters are necessary only in high altitude photography, although some people keep them on all the time, to protect the front element of their lens.
When taking pictures of some flowers, you may want to mount a UV filter in front of your electronic flash: the strobe sends out UV, and certain flowers have vegetal dyes that exhibit fluorescence (they convert UV to visible lights, just like fluorescent lights), leading to bizarre colors in the picture.
As a final note, remember that unscrupulous manufacturers have been known to sell pieces of untreated glass as UV filters (they look exactly the same). If you buy one, get a good brand, like Tiffen or B+W.