Learner's definition of THRESHOLD
1 : a piece of wood, metal, or stone that forms the bottom of a door and that you walk over as you enter a room or building 2 : the point or level at which something begins or changes
If your income rises above a certain threshold, your tax rate also rises.
sounds that are above/below the threshold of hearing [=sounds that are too high/low to be heard]
I have a low threshold for boredom. [=I get bored easily]
He has a high pain threshold. [=he does not feel pain as easily as other people]
on/at the threshold
: at the beginning of something or very close to something (such as a new condition, an important discovery, etc.) — usually + of