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.
Phrases & Compounds
Sill course
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.