A treadmill is a piece of indoor sporting equipment used to allow for the motions of running or walking while staying in one place. The word treadmill traditionally refers to a type of mill which was operated by a person or animal treading steps of a wheel to grind grain (see treadwheel.)

The principle is a belt system (an electric motor or flywheel connected to a circular run of material) where the top of the belt moves to the rear so as to allow a runner to run an equal, and necessarily opposite speed. Thus the speed of the run can easily be set or measured (the rate at which the belt moves is the rate at which the runner moves). The more expensive, heavy-duty versions are motor-driven. The simpler, lighter, cheaper versions are passive, moving only when the walker pushes the belt with their feet, and operates just to resist the motion.