1. structural member consisting of a continuous horizontal timber forming the lowest member of a framework or supporting structure
2. (geology) a flat (usually horizontal) mass of igneous rock between two layers of older sedimentary rock
3. The basis or foundation of a thing; especially, a horizontal piece, as a timber, which forms the lower member of a frame, or supports a structure; as, the sills of a house, of a bridge, of a loom, and the like.