Source : Webster's Revised Unabridged Dictionary (1913)

Doorsill \Door"sill`\, n.
   The sill or threshold of a door.

Source : WordNet®

doorsill
     n : 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: {doorstep}, {threshold}]