the entrance to a room or building, or the area of floor or ground at the entrance:
She opened the door and stepped across the threshold.
the level at which something starts to happen or have an effect:
Eighty percent of the vote was the threshold for approval of the plan.
a high/low pain/boredom etc threshold (=the ability or inability to suffer a lot of pain or boredom before you react to it)
at the beginning of a new and important event or development
be on the threshold of something
The creature is on the threshold of extinction.