Real Dictionary

threshold
a horizontal datum line or plane [...]
a recurring period of time in which certain events or phenomena occur and reach completion or repeat themselves in a regular sequence [...]
a sill, esp one made of stone or hardwood, placed at a doorway [...]
A threshold is an amount, level, or limit on a scale. When the threshold is reached, something else happens or changes.
the desirability of a thing, often in respect of some property such as usefulness or exchangeability; worth, merit, or importance [...]
The threshold of a building or room is the floor in the doorway, or the doorway itself.