(noun)

1. a. The piece of timber or stone which lies below the bottom of a door, and has to be crossed in entering a house; the sill of a doorway; hence, the entrance to a house or building.

2. transf. and fig. a. Border, limit (of a region); the line which one crosses in entering.
b. In reference to entrance, the beginning of a state or action, outset, opening.
c. In technical language, a lower limit. (i) Psychol.: esp. in phr. threshold of consciousness, cf. LIMEN, SUBLIMINAL. In Physiol. and more widely: the limit below which a stimulus is not perceptible; the magnitude or intensity of a stimulus which has to be exceeded for it to produce a certain response. (ii) The magnitude or intensity that must be exceeded for a certain reaction or phenomenon to occur.