Threshold :  (noun)
1: the starting point for a new state or experience; "on the threshold of manhood"
2: the smallest detectable sensation [syn: limen]
3: the entrance (the space in a wall) through which you enter or leave a room or building; the space that a door can close; "he stuck his head in the doorway" [syn: doorway, door, room access]
4: the sill of a door; a horizontal piece of wood or stone that forms the bottom of a doorway and offer support when ... ( more )

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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,