WordNet sense 1 (a region marking a boundary):

brink, verge

WordNet sense 2 (the smallest detectable sensation):

limen

WordNet sense 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):

door, doorway, room access

WordNet sense 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):

doorstep, doorsill