Definitions of threshold:

• noun:   the smallest detectable sensation
• noun:   the starting point for a new state or experience

  Example: "On the threshold of manhood"

• noun:   a region marking a boundary
• noun:   the sill of a door; a horizontal piece of wood or stone that forms the bottom of a doorway and offer support when passing through a doorway
• noun:   the entrance (the space in a wall) through which you enter or leave a room or building; the space that a door can close