n.1 a strip of wood or stone forming the bottom of a doorway and crossed in entering a house or room etc.
2 a point of entry or beginning (on the threshold of a new century).
3 Physiol. & Psychol. a limit below which a stimulus causes no reaction (pain threshold).
4 Physics a limit below which no reaction occurs, esp. a minimum dose of radiation producing a specified effect.
5 (often attrib.) a step in a scale of wages or taxation, usu. operative in specified conditions.
Etymology: OE therscold, threscold, etc., rel. to THRASH in the sense 'tread'