thresh·old 
(thrĕshōld′, -hōld′)
(thrĕshōld′, -hōld′)n.
1. A piece of wood or stone placed beneath a door; a doorsill.
2. Either end of an airport runway.
3. The place or point of beginning; the outset: on the threshold of a new era.
4. The point that must be exceeded to begin producing a given effect or result or to elicit a response: a low threshold of pain.
[Middle English thresshold, from Old English therscold, threscold; see terə-1 in the Appendix of Indo-European roots.]