A device, used by graphic artists and photographers to view transparent/translucid material.
The light box is composed of a light mixing chamber, usually lined with white or metallic reflectors, and a white translucent sheet of plastic or glass.
The chamber contains light sources, usually high quality fluorescent tubes (a CRI of over 90 is reccomended).
In a properly designed light box, the top sheet of translucent material is evenly lit (you can't tell where the tubes are) with a steady, uniform glow.

      material for viewing goes here
|                                        |
| ======fluorescent=lighting============ |

Light boxes go from a miniature size that allows viewing of some 35 mm slides to much bigger units (I have seen even A2, like four A4s). The really big ones sprout legs and turn into light tables.
Things to look for in a light box: you want a reasonable light intensity (but anyway you will end up having to adjust the ambient light levels), the surface must be uniformly lit, and the fluorescents must be high quality, particularly for watching and editing slides.