A
treadwheel, or
treadmill, is a form of
engine typically powered by humans. It may resemble a
water wheel in appearance, and can be worked either by a human treading paddles set into its circumference (treadmill), or by a human or animal standing inside it (treadwheel). These devices are no longer used for power or punishment, and the term "
treadmill" has come to mean an
exercise machine for running or walking in place.