Definitions
1.nounThe bottom-most part of a doorway that one crosses to enter; a sill.
2.noun(by extension) An entrance; the door or gate of a house.
3.noun(by extension) Any end or boundary.
4.nounThe outset of something; the point of entry, or the beginning of an action.
5.nounThe start of the landing area of a runway.
6.nounThe quantitative point at which an action is triggered, especially a lower limit.
7.nounThe wage or salary at which income tax becomes due.
8.nounThe point where one mentally or physically is vulnerable in response to provocation or to particular things in general. As in emotions, stress, or pain.