Sill
WordNet
noun
(1) Structural member consisting of a continuous horizontal timber forming the lowest member of a framework or supporting structure
(2) (geology) a flat (usually horizontal) mass of igneous rock between two layers of older sedimentary rock
WiktionaryText
Noun
- A horizontal slat which forms the base of a window.
- She looked out the window resting her elbows on the window sill.
- A horizontal member bearing the upright portion of a frame.
- A horizontal layer of igneous rock between older rock beds.
Usage notes
Usually spelled cill when used in the context of canal or river engineering.