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]