From Wikipedia, the free encyclopedia
A window-sill is:
- The lowest part of a window frame. Window sills hold the side pieces in place and slope outward to drain water. In a double hung window, the lower sash rests on the sill;
- The horizontal piece below a window unit in masonry construction or in wood framing. The window sill of the window frame sits on the window sill of the wall opening;
- The lowermost, interior trim work on a window;
- Meets the dictionary definition of a shelf "A flat, usually rectangular structure composed of a rigid material, such as wood, glass, or metal, fixed at right angles to a wall or other vertical surface and used to hold or store objects." 
- Sturgis, Russell. A dictionary of architecture and building: biographical, historical, and descriptive , vol.3. New York: The Macmillan Co. ;, 1901. 1,100. Print.