1. An exercise machine that has a large flat belt that moves while someone walks or runs on it: Whenever Klaus went to the fitness studio, he spent about forty-five minutes running on the treadmill.

The treadmill was invented and named in 1822 by William Cubitt of Ipswich, England; and it was originally an instrument used for prison discipline.

2. A similar exercise mechanism that is operated by quadrupeds; such as, horses: The dog owner had a treadmill for her dog so it could get more exercise in the city apartment.
3. A boring or tiring activity, job, etc., in which a person repeatedly does the same things again and again: The office treadmill and the treadmill of exhausting family schedules made it necessary for Aileen to go on a vacation.

Gavin was getting tired of working on the project treadmill because no matter how much effort he and the others put into it, nothing was being accomplished.

4. A system consisting of a large wheel with steps fitted into its inner surface which was formerly used for operating machinery: Some previous treadmills were turned by the weight of people who were treading on steps so the equipment could manufacture products.