threshold
This usually means the doorway of a building.
Threshold originally meant the wooden board at the bottom of a doorway: the board was used to keep the 'thresh' (the straw used to cover the floor of the house) from being tracked out of the house.
This photo shows the open doorway of an old house. The board at the bottom of the doorway is the threshold.http://commons.wikimedia.org/wiki/File:Plimoth_Plantation_Fence.JPG