sill

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The horizontal member that bears the upright portion of a frame, especially the horizontal member that forms the base of a window.
2. n. Geology An approximately horizontal sheet of igneous rock intruded between older rock beds.

Century Dictionary and Cyclopedia

1. n. A stone or piece of timber on which a structure rests; a block forming a basis or foundation: as, the sills of a house, of a bridge, of a loom; more specifically, a horizontal piece of timber of the frame of a building, or of wood or stone at the bottom of a framed case, such as that of a door or window; in absolute use, a door-sill. See door-sill, ground-sill, mudsill, port-sill, window-sill.
2. n. In fortification, the inner edge of the bottom or sole of an embrasure. See diagram under embrasure.
3. n. In mining: The floor of a gallery or passage in a mine.
4. n. A term used by miners in the lead districts of the north of England as nearly equivalent to bed or stratum. Thus, the basaltic sheets intercalated in the mountain-limestone are called whin -sills.
5. n. A young herring.
6. n. A variant of sell.
7. n. The thill or shaft of a carriage.

Wiktionary

1. n. A horizontal slat which forms the base of a window.
2. n. A horizontal, structural member of a building near ground level on a foundation (sense #3) or pilings or lying on the ground in earth-fast construction and bearing the upright portion of a frame. Also spelled cill. Also called a ground plate, groundsill, sole, sole-plate, mudsill. An interrupted-sill fits between posts instead of being below and supporting the posts in timber framing.
3. n. geology A horizontal layer of igneous rock between older rock beds.
4. n. UK A young herring.

GNU Webster's 1913

1. n. The timber or stone at the foot of a door; the threshold.
2. n. The timber or stone on which a window frame stands; or, the lowest piece in a window frame.
3. n. The floor of a gallery or passage in a mine.
4. n. A piece of timber across the bottom of a canal lock for the gates to shut against.
5. n. Prov. Eng. The shaft or thill of a carriage.
6. n. engraving A young herring.

WordNet 3.0

1. n. (geology) a flat (usually horizontal) mass of igneous rock between two layers of older sedimentary rock
2. n. structural member consisting of a continuous horizontal timber forming the lowest member of a framework or supporting structure