noun

• (historical) A mill worked by persons treading upon steps on the periphery of a wide wheel having a horizontal axis. It was used principally as a means of prison discipline.
  "Their paternal house of Drummington, Foker could very seldom be got to visit. He swore he had rather go on the treadmill than stay there."
• (figurative) A process or situation in which continued effort leads to or is required for remaining at a particular state or level without moving ahead.
  "1 As a result, people can easily find themselves alternating between various treadmills: the hedonic treadmill of pursuing happiness, the status treadmill requiring conspicuous consumption, and the treadmill of work undertaken to finance one's activity on the other two treadmills."

verb

• (intransitive) To exercise on a treadmill.