A sill plate or sole plate in construction and architecture is the bottom horizontal member of a wall or building to which vertical members are attached. The word plate is typically omitted in America and carpenters speak simply of the “sill.” Other names are ground plate, ground sill, groundsel, and midnight sill. Sill plates are usually composed of lumber but can be any material. The timber at the top of a wall is often called a top plate, pole plate, wall plate or simply “the plate.”

Description sourced from Wikipedia. Image sourced from UpCodes.