n.
A simple machine consisting of an axle to which a wheel is fastened so that torque applied to the wheel winds a rope or chain onto the axle, yielding a mechanical advantage equal to the ratio of the diameter of the wheel to that of the axle.
In other words:
A simple machine consisting of an axle, the pin or spindle on which a wheel revolves, to which a wheel is put on so that a turning or twisting force is applied to the wheel.
Links
Definition and Description of the Wheel and Axle
Examples of the Wheel and Axle (Pictures)
Uses of the Wheel and Axle
Interesting Facts and Trivia about the Wheel and Axle