threshold

the starting point for a new state or experience; "on the threshold of manhood"

the smallest detectable sensation

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"

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

a region marking a boundary