From Wikipedia, the free encyclopedia
A window-sill is:
- The lowest part of a window frame. The window sills slope outward to drain water and hold the side pieces in place. 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.
- This is not a shelf and should not be used for storage.
- 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." 
Difference between sill and cill
British spelling of "sill" is "cill".
- Sturgis, Russell. A dictionary of architecture and building: biographical, historical, and descriptive , vol.3. New York: The Macmillan Co. ;, 1901. 1,100. Print.