Sill (noun)

1. A horizontal piece of wood, stone, or metal fixed across the bottom of a window or door frame to provide a surface for the window or door to close against.
2. The flat piece that forms the lower edge of an opening such as a window, door or fireplace.

Origin:

Old english sylle "strip of wood or stone placed under the frame of a window or door", of germanic origin.

Examples:

1. The window sill was painted white.
2. The cat was sleeping on the window sill.
3. The door sill was worn and needed to be replaced.
4. The window sill was a good place to grow herbs.
5. The sill on the door was made of marble.