Noun

(en noun)

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

(by extension) An entrance

The start of the landing area of a runway

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

The wage or salary at which income tax becomes due

The outset of an action or project

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.

The point of beginning or entry

From all the pressure my partner has been through lately, his emotion threshold has suddenly gotten pretty low these days. I can tell because he easily loses it when he is around people or hears about anything to do with his concerns.

Noun

(en noun)

The beginning of something.

The source of a river, information, goods, etc.

* {{quote-magazine, date=2013-06-14, author= Sam Leith

, volume=189, issue=1, page=37, magazine=(The Guardian Weekly) , title= Where the profound meets the profane , passage=Swearing doesn't just mean what we now understand by "dirty words". It is entwined, in social and linguistic history, with the other sort of swearing: vows and oaths. Consider for a moment the origins of almost any word we have for bad language – "profanity", "curses", "oaths" and "swearing" itself.}}

(mathematics) The point at which the axes of a coordinate system intersect.

(anatomy) The proximal end of attachment of a muscle to a bone that will not be moved by the action of that muscle.

(cartography) An arbitrary point on the earth's surface, chosen as the zero for a system of coordinates.

(in the plural) Ancestry.