A treadmill is an exercise machine with a moving belt you walk or run on while staying in one place. The belt rotates under your feet, powered by a motor or your own movement, so you can run for miles without going anywhere. It's like walking up a down escalator: you keep moving, but you stay put.

Treadmills let people exercise indoors regardless of weather, darkness, or traffic. Gyms fill entire rooms with them, and many families keep one at home. You can adjust the speed to walk slowly or sprint hard, and many treadmills let you increase the incline to simulate running uphill.

The word also describes any repetitive, seemingly endless routine that feels like you're working hard but not getting anywhere. When someone complains about being on a treadmill at school or home, they mean they're stuck in a boring cycle: wake up, do the same things, repeat, with no sense of progress or change. A student might feel like they're on a treadmill when studying the same material over and over without improving. This metaphorical meaning captures that frustrating feeling of constant effort without forward movement, just like the exercise machine.