The Collaborative International Dictionary of English v.0.48:

Threshold \Thresh"old\, n. [OE. threswold, [thorn]reshwold, AS.
   [thorn]rescwald, [thorn]erscwald, [thorn]erscold,
   [thorn]rescold, fr. [thorn]rescan, [thorn]erscan, to thresh;
   akin to Icel. [thorn]reskj["o]de, [thorn]r["o]skuldr, Sw.
   tr["o]skel, Dan. t[ae]rskel. See Thrash.]
   1. 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.
      [1913 Webster]

   2. Fig.: The place or point of entering or beginning,
      entrance; outset; as, the threshold of life.
      [1913 Webster]

WordNet (r) 3.0 (2006):

threshold
    n 1: the starting point for a new state or experience; "on the
         threshold of manhood"
    2: the smallest detectable sensation [syn: threshold, 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, threshold]
    4: the sill of a door; a horizontal piece of wood or stone that
       forms the bottom of a doorway and offers support when passing
       through a doorway [syn: doorsill, doorstep, threshold]
    5: a region marking a boundary [syn: brink, threshold,
       verge]

Moby Thesaurus II by Grady Ward, 1.0:

107 Moby Thesaurus words for "threshold":
   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



Easton's 1897 Bible Dictionary:

Threshold
   (1.) Heb. miphtan, probably a projecting beam at a higher point
   than the threshold proper (1 Sam. 5:4,5; Ezek. 9:3; 10:4,18;
   46:2; 47:1); also rendered "door" and "door-post."

     (2.) 'Asuppim, pl. (Neh. 12:25), rendered correctly
   "storehouses" in the Revised Version. In 1 Chr. 26:15, 17 the
   Authorized Version retains the word as a proper name, while in
   the Revised Version it is translated "storehouses."