The Collaborative International Dictionary of English v.0.48:

Doorsill \Door"sill`\, n.
   The sill or threshold of a door.
   [1913 Webster]

WordNet (r) 3.0 (2006):

doorsill
    n 1: 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]