noun (n)

• the starting point for a new state or experience(noun.time)

  Example:
  On the threshold of manhood.
  

  source: wordnet30
• the smallest detectable sensation(noun.cognition)

  Synonym:
  limen
  

  source: wordnet30
• the entrance (the space in a wall) through which you enter or leave a room or building; the space that a door can close(noun.artifact)

  Synonym:
  door, doorway, room access
  

  source: wordnet30
• 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(noun.artifact)

  Synonym:
  doorsill, doorstep
  

  source: wordnet30
• a region marking a boundary(noun.shape)

  Synonym:
  brink, verge
  

  source: wordnet30
• The plank, stone, or piece of timber, which lies under a door, especially of a dwelling house, church, temple, or the like; the doorsill; hence, entrance; gate; door.(noun)

  source: webster1913