treadmill

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A wheel with steps on which a person treads, used to power machinery.
• noun An exercise device consisting of a continuous moving belt on which a person or animal can walk or run while remaining in one place.
• noun A monotonous routine or set of tasks.

from The Century Dictionary.

• noun An appliance for producing rotary motion by the weight of a man or men, or of an animal, as a horse, stepping on movable steps connected with a revolving cylinder or wheel.
• noun Figuratively, a monotonous and wearisome round, as of occupation or exertion: as, the treadmill of business.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A piece of indoor sporting equipment used to allow for the motions of running or walking while staying in one place.
• noun 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.
• noun A mill worked by horses, dogs, etc., treading an endless belt.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an exercise device consisting of an endless belt on which a person can walk or jog without changing place
• noun a job involving drudgery and confinement
• noun a mill that is powered by men or animals walking on a circular belt or climbing steps