sill

1. (architecture, also "window sill") A breast wall; window breast; horizontal brink which forms the base of a window.Examples: "She looked out the window resting her elbows on the window sill."

2. (construction) A threshold; horizontal structural member of a building near ground level on a foundation or pilings, or lying on the ground, and bearing the upright portion of a frame; a sill plate.

3. (geology) A stratum of rock, especially an intrusive layer of igneous rock lying parallel to surrounding strata.Examples: "Minor palingenetic magmas probably were generated at this time and intruded the mantling rocks in the form of small sills and apophyses […]."; "The molten rock in the sills may have ignited vast reserves of shallowly buried natural gas, much like a match applied to a gas barbecue."

4. (anatomy) A raised area at the base of the nasal aperture in the skull.Examples: "the nasal sill"

5. (military, historical) The inner edge of the bottom of an embrasure.historical