threshold

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A piece of wood or stone placed beneath a door; a doorsill.
2. n. An entrance or a doorway.
3. n. The place or point of beginning; the outset.
4. n. The point that must be exceeded to begin producing a given effect or result or to elicit a response: a low threshold of pain.

Century Dictionary and Cyclopedia

1. n. The plank, stone, or piece of timber which lies at the bottom of a door, or under it, particularly the door of a dwelling-house, church, temple, or other building; a door-sill; hence, entrance; gate; door.
2. n. Hence, the place or point of entering or beginning; outset: as, he is now at the threshold of his argument.
3. n. In psychology, the limit below which a given stimulus,' or the difference between two stimuli, ceases to be perceptible. Compare schwelle.

Wiktionary

1. n. The bottom-most part of a doorway that one crosses to enter; a sill.
2. n. An entrance
3. n. The start of the landing area of a runway
4. n. The quantitative point at which an action is triggered, especially a lower limit
5. n. The wage or salary at which income tax becomes due
6. n. The outset of an action or project
7. n. The point where one mentally or physically is vulnerable in response to provocation or to particular things in general. As in emotions, stress, or pain.

GNU Webster's 1913

1. n. The plank, stone, or piece of timber, which lies under a door, especially of a dwelling house, church, temple, or the like; the doorsill; hence, entrance; gate; door.
2. n. Fig.: The place or point of entering or beginning, entrance; outset.

WordNet 3.0

1. n. the entrance (the space in a wall) through which you enter or leave a room or building; the space that a door can close
2. n. the smallest detectable sensation
3. n. a region marking a boundary
4. n. the sill of a door; a horizontal piece of wood or stone that forms the bottom of a doorway and offers support when passing through a doorway
5. n. the starting point for a new state or experience