The Collaborative International Dictionary of English v.0.48:

limen \limen\ n. (Psychol.)
   a threshhold, especially the point where a psychological or
   physiological effect begins to occur.
   [PJC]

WordNet (r) 3.0 (2006):

limen
    n 1: the smallest detectable sensation [syn: threshold,
         limen]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "limen":
   affectibility, border line, bound, boundary, boundary condition,
   boundary line, bourn, break boundary, breakoff point, ceiling,
   circumscription, compass, confine, cutoff, cutoff point, deadline,
   delimitation, determinant, division line, end, extremity, finish,
   floor, frontier, hedge, high-water mark, impressibility,
   impressionability, interface, limit, limitation, limiting factor,
   line, line of demarcation, low-water mark, lower limit, march,
   mark, mete, openness to sensation, perceptibility,
   physical sensibility, readiness of feeling, receptiveness,
   receptivity, sensation level, sensibility, sensibleness, sentience,
   sentiency, start, starting line, starting point, susceptibility,
   susceptivity, target date, term, terminal date, terminus,
   threshold, threshold of sensation, time allotment, upper limit