The Collaborative International Dictionary of English v.0.48:

Treadmill \Tread"mill`\, n.
   A mill worked by persons treading upon steps on the periphery
   of a wide wheel having a horizontal axis. It is used
   principally as a means of prison discipline. Also, a mill
   worked by horses, dogs, etc., treading an endless belt.
   [1913 Webster]

WordNet (r) 3.0 (2006):

treadmill
    n 1: an exercise device consisting of an endless belt on which a
         person can walk or jog without changing place
    2: a mill that is powered by men or animals walking on a
       circular belt or climbing steps [syn: treadmill,
       treadwheel, tread-wheel]
    3: a job involving drudgery and confinement [syn: treadmill,
       salt mine]