What is cadence?

In cycling, cadence is the number of revolutions of the crank per minute; roughly speaking, this is the speed at which a cyclist is pedalling/turning the pedals.

Cyclists typically have a preferred cadence at which they feel most comfortable, and on bicycles with many gears it is possible to stick to a favourite cadence at a wide range of speeds. Recreational and utility cyclists typically cycle around 60?80 rpm; racing cyclists around 80?120 rpm and sprinters up to 170 rpm for short bursts. The professional racing cyclist and Tour de France champion Lance Armstrong is known for his technique of keeping up high cadences of around 110 rpm for hours on end to improve efficiency.
Some cyclists believe that some cadences are more efficient than others, but the wide range of preferred cadences among racing cyclists suggest that the difference, if any, is small.
An important point is that any particular cyclist has only a narrow range of preferred cadences, often smaller than the general ranges listed above. This in turn influences the number and range of gears which are appropriate for any particular cycling conditions.

Source: http://en.wikipedia.org/wiki

Speed/Cadence bike sensor (GSC 10)