tread ‧ mill /ˈtredˌmɪl/ BrE AmE noun
1 . [countable] a piece of exercise equipment that has a large belt around a set of wheels, that you can walk or run on while staying in the same place
2 . [singular] work or a way of life that seems very boring because you always have to do the same things:
the treadmill of working in the office
3 . [countable] a ↑ mill worked in the past by prisoners treading on steps fixed to a very large wheel