Merriam-Webster Collegiate Dictionary - threshold

 noun  Etymology: Middle English thresshold, from Old English threscwald; akin to Old Norse threskjoldr ~, Old English threscan to thresh  Date: before 12th century  1. the plank, stone, or piece of timber that lies under a door ; sill  2.  a. gate, door  b.  (1) end, boundary; specifically the end of a runway  (2) the place or point of entering or beginning ; outset on the ~ of a new age  3.  a. the point at which a physiological or psychological effect begins to be produced has a high ~ for pain  b. a level, point, or value above which something is true or will take place and below which it is not or will not