threshold

(noun) the starting point for a new state or experience. Example: "On the threshold of manhood."

(noun) the smallest detectable sensation. Synonyms: limen.

(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. Synonyms: doorway, door, room access. Example: "He stuck his head in the doorway."

(noun) 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.

(noun) a region marking a boundary. Synonyms: brink, verge.