Threshold
Etymology
From Old English therscold, the strip of wood or stone at the foot of a doorway that you step over to enter. Its deeper origins are uncertain, though it is linked to "thresh," the act of trampling grain. Science kept the crossing-a-line image for any point at which a system’s response abruptly begins.
noun · /THRESH-hohld/
The level a quantity must reach before an effect suddenly switches on, with little or nothing happening below it.
What it names
Why pressure can build for a long time with no visible result and then produce an abrupt change all at once, once a critical level is crossed.
Example
Her patience had a threshold: a dozen small interruptions did nothing, and then the thirteenth set her off completely.
Related idea
A threshold is the trigger level for an effect; criticality is the special state right at such a point, where a system is poised to tip and small nudges have outsized results.
noun · /THRESH-hohld/
The strip of floor at the bottom of a doorway; the entrance itself.
What it names
the literal doorsill one steps over to enter a room or building
Example
He carried her over the threshold into the new house.
Related idea
The “level at which an effect begins” is this doorway made abstract: to cross a threshold is to pass from outside a change to inside it — to stand on the cusp.