What is the meaning of threshold?

noun

• The starting point for a new state or experience.
  • usage: "on the threshold of manhood"
• The smallest detectable sensation.
  • synonyms: limen
• The entrance (the space in a wall) through which you enter or leave a room or building; the space that a door can close.
  • usage: "he stuck his head in the doorway"
  • synonyms: doorway, door, room access
• 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.
  • synonyms: doorsill, doorstep
• A region marking a boundary.
  • synonyms: brink, verge