From the glossary
Window
A pale, see-through area at the centre of a stone's face-up view, where light passes straight through instead of returning to the eye. The cause is a pavilion cut too shallow, often to save weight from flat rough. A windowed stone carries carats that add weight but not colour. Recutting closes a window at a cost in weight, and whether that trade pays is a calculation a good dealer will do with you openly.