From Longman Dictionary of Contemporary Englishtreadmilltread‧mill /ˈtredˌmɪl/ 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 place2 [singular]BORING 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 office3 [countable]TP a mill worked in the past by prisoners treading on steps fixed to a very large wheel
[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 place2 [singular]BORING 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 office3 [countable]TP a mill worked in the past by prisoners treading on steps fixed to a very large wheelExamples from the Corpus
treadmill• Join a gym or buy a treadmill?• Each peahen is on a treadmill and dare not jump off lest she condemn her sons to celibacy.• It has been described as bare-knuckle fighting without a ring and like setting toy mice on a treadmill.• Here was a treadmill on which trainees got launched, often too quickly.• Width often leads to superficiality and depth may produce a blinkered approach and an intellectual treadmill.• An intensely private man, one can only assume the attractions of the year-round treadmill are wearing thin.• And ask any worker about the treadmill, the maddening tedium.• He ran two miles on the treadmill. 