Definition of: sill

(sil) noun
1. A horizontal member forming the foundation, or part of the foundation, of a structure of any kind, as at the bottom of a casing in a building; especially, a door sill or a window sill.
2. A timber in the frame of the floor of a railroad car: end sill; side sill.
3. Geol. A relatively thin stratum of igneous rock intruded between level or gently inclined beds of other rock. [OE syll]