treadmill

Concise Oxford English Dictionary © 2008 Oxford University Press:
treadmill/ˈtredmɪl/

▶noun

• 1 (also treadwheel) a large wheel turned by the weight of people or animals treading on steps fitted into its inner surface, formerly used to drive machinery.

• 2 a device used for exercise consisting of a continuous moving belt on which to walk or run.

• 3 a job or situation that is tiring, boring, or unending.