From The Collaborative International Dictionary of English v.0.48 :
Doorsill \Door"sill`\, n.
The sill or threshold of a door.
[1913 Webster]
From 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]