threshold

noun

1. The bottom-most part of a doorway that one crosses to enter; a sill.

2. (by extension) An entrance; the door or gate of a house.

3. (by extension) Any end or boundary.

4. The outset of something; the point of entry, or the beginning of an action.

5. The start of the landing area of a runway.

6. The quantitative point at which an action is triggered, especially a lower limit.

7. The wage or salary at which income tax becomes due.

8. The point where one mentally or physically is vulnerable in response to provocation or to particular things in general. As in emotions, stress, or pain.