(n.) The stone forming a threshold.

base basement basis bearing wall bed bedding bedrock doorsill doorstone floor flooring fond footing foundation fundament fundamental ground grounds groundsel groundwork hardpan mudsill pavement principle radical riprap rock bottom rudiment seat solid ground solid rock stereobate stylobate substratum substruction substructure terra firma threshold underbuilding undercarriage undergirding underpinning understruction understructure window sill