Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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: {treadwheel}, {tread-wheel}]
     3: a job involving drudgery and confinement [syn: {salt mine}]