threshold

noun

1. the starting point for a new state or experience"on the threshold of manhood"
2. the smallest detectable sensation
3. the entrance (the space in a wall) through which you enter or leave a room or building; the space that a door can close"he stuck his head in the doorway"
4. 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
5. a region marking a boundary