1. (n.) The plank, stone, or piece of timber, which lies under a door, especially of a dwelling house, church, temple, or the like; the doorsill; hence, entrance; gate; door.

2. (n.) Fig.: The place or point of entering or beginning, entrance; outset; as, the threshold of life.

French door archway back door barway beginning border line bound boundary boundary condition boundary line bourn break boundary breakoff point brink bulkhead carriage entrance ceiling cellar door cellarway circumscription commencement compass confine cutoff cutoff point dawn deadline delimitation determinant division line door doorjamb doorpost doorsill doorstep doorstone doorway edge end entrance entrance hall entranceway entry entryway extremity finish floor foyer front door frontier galilee gate gatepost gateway groundsel hatch hatchway hedge high-water mark interface limen limit limitation limiting factor line line of demarcation lintel lobby low-water mark lower limit march mark mete mudsill narthex outset point porch portal porte cochere portico postern propylaeum pylon scuttle side door sill start starting line starting point stile stoa storm door target date term terminal date terminus time allotment tollgate trap trap door turnpike turnstile upper limit verge vestibule window sill