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.


photo of a doorway
The threshold of an old home
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


thresholdauthor