SILL

noun Etymology: Middle English ~e, from Old English syll; akin to Old High German swelli beam, threshold Date: before 12th century a horizontal piece (as a timber) that forms the lowest member or one of the lowest members of a framework or supporting structure: as, the horizontal member at the base of a window, the threshold of a door, a tabular body of igneous rock injected while molten between sedimentary or volcanic beds or along foliation planes of metamorphic rocks, a submerged ridge at relatively shallow depth separating the basins of two bodies of water