threshold

noun

1. 1

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

2. 2

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

3. 3

   (by extension) Any end or boundary.

4. 4

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

5. 5

   The start of the landing area of a runway.

6. 6

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

7. 7

   The wage or salary at which income tax becomes due.

8. 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.