treadmill

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A mechanism rotated by people treading on the moving steps of a wheel.
2. n. A similar device operated by an animal treading a continuous sloping belt.
3. n. An exercise device consisting of a continuous moving belt on which a person can walk or jog while remaining in one place.
4. n. A monotonous task or set of tasks seeming to have no end.

Century Dictionary and Cyclopedia

1. n. 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. The name is now rarely given to industrial appliances of this nature, but chiefly to those used as means of punishment in some prisons. Compare horse-power, 3, and see cut in next column.
2. n. Figuratively, a monotonous and wearisome round, as of occupation or exertion: as, the treadmill of business.

Wiktionary

1. n. A piece of indoor sporting equipment used to allow for the motions of running or walking while staying in one place.
2. 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.
3. n. A mill worked by horses, dogs, etc., treading an endless belt.

GNU Webster's 1913

1. 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.

WordNet 3.0

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