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]

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "treadmill":
   beat, beaten path, branks, broken record, bureaucracy,
   bureaucratism, chinoiserie, clockwork regularity, constancy, crank,
   cucking stool, daily grind, daily round, dingdong, dirty work,
   donkeywork, drone, drudgery, ducking stool, employment, even pace,
   even tenor, fag, fatigue, finger pillory, grind, groove, handiwork,
   handwork, humdrum, industry, invariability, irk, irksomeness,
   jog trot, labor, lick, lick of work, manual labor, moil, monologue,
   monotone, monotonousness, monotony, orderliness, pace, pillory,
   rat race, red tape, red-tapeism, regularity, rote, round, routine,
   run, rut, sameliness, sameness, scut work, singsong, slavery,
   smoothness, spadework, squirrel cage, stocks, stroke,
   stroke of work, sweat, task, tedium, the beaten track,
   the daily round, the round, the squirrel cage, the treadmill,
   the weary round, tiresome work, toil, track, travail, trebuchet,
   triangle, triangles, undeviation, undifferentiation, unvariation,
   wearisome sameness, well-worn groove, whipping post, wooden horse,
   work