Sill n.  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. Hence: (a) The timber or stone at the foot of a door; the threshold. (b) The timber or stone on which a window frame stands; or, the lowest piece in a window frame. (c) The floor of a gallery or passage in a mine. (d) A piece of timber across the bottom of a canal lock for the gates to shut against.
 Sill course Arch., a horizontal course of stone, terra cotta, or the like, built into a wall at the level of one or more window sills, these sills often forming part of it.