treadmill

noun

1. a kind of mill wheel turned by the weight of persons treading steps arranged around its circumference: formerly used as an instrument of prison discipline
2. a mill driven by an animal treading a sloping, endless belt
3. any monotonous round of duties, work, etc. in which one seems to get nowhere
4. a stationary exercise machine, often with speed controls, handles, etc., consisting of a short, wide conveyor belt on which a person walks or runs