Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Source : WordNet®

threshold
     n 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
        passing through a doorway [syn: {doorsill}, {doorstep}]
     5: a region marking a boundary [syn: {brink}, {verge}]