threshold

Concise Oxford English Dictionary © 2008 Oxford University Press:
threshold /ˈθrɛʃəʊld, ˈθrɛʃˌhəʊld/

▶noun

• 1 a strip of wood or stone forming the bottom of a doorway and crossed in entering a house or room.

• 2 a level or point at which something starts or ceases to happen or come into effect.

  ■ the level at which one starts to feel or react to something: a low pain threshold.

– origin OE therscold, threscold; first element rel. to thresh (in the sense ‘tread’).