threshold

noun

1. The lowermost part of a doorway that one crosses to enter; a sill.
2. (broadly)An entrance; the door or gate of a house.

   “Shy. You that did voide your rume vpon my beard, / And foote me as you ſpurne a ſtranger curre / Ouer your threſhold, […]”

3. (broadly)Any end or boundary.
4. (figuratively)The outset of something; the point of entry, or the beginning of an action.

   “I arrived at last, did obeisance to my uncle, and told him everything. He thought it over and said: ' […] At the threshold of death, how dare I give you permission to go to England, to cross the seas? But I will not stand in your way. It is your mother's permission which really matters. If she permit you, then godspeed! Tell her I will not interfere. You will go with my blessings.'”

   “The first half of this century has been referred to as the golden age of medicine. To me it seems more probable that we are on the threshold of a much greater age.”

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 is mentally or physically vulnerable in response to a provocation or to other nuisances.