[sill] n [ME sille, fr. OE syll; akin to OHG swelli beam, threshold] (bef. 12c) 1: a horizontal piece (as a timber) that forms the lowest member or one of the lowest members of a framework or supporting structure: as a: the horizontal member at the base of a window b: the threshold of a door
2: a tabular body of igneous rock injected while molten between sedimentary or volcanic beds or along foliation planes of metamorphic rocks
3: a submerged ridge at relatively shallow depth separating the basins of two bodies of water