An area of the faceted diamond that allows light to leak out rather than reflecting and refracting it back to another facet. In gemstones, a window refers to the pavilion of the gemstone which appears transparent, also known as a "fish eye". When viewed from the top (the table), the gemstone appears to have very little faceting on the bottom; usually this issue can be remedied by mounting in a ring or pendant setting.
See Also: Cut, Make, Table, Culet