A sprocket[1] or sprocket-wheel[2] is a profiled wheel with tooth, or cogs,[3][4] that mesh with a chain, track or other perforated or indented material.[5][6] The name ‘sprocket’ applies generally to any wheel where radial projections engage a chain passing over it. It is distinguished from a gear in that sprockets should never be meshed together straight, and differs from a pulley in that sprockets have the teeth and pulleys are smooth.

Sprockets are found in bicycles, motorcycles, cars, tracked vehicles, and other machinery either to transmit rotary motion between two shafts where gears are unsuitable or to impart linear motion to a track, tape etc. Maybe the most typical form of sprocket could be found in the bicycle, in which the pedal shaft carries a huge sprocket-wheel, which drives a chain, which, in turn, drives a little sprocket on the axle of the trunk wheel. Early automobiles had been also largely driven by sprocket and chain mechanism, a practice mainly copied from bicycles.

Sprockets are of varied designs, no more than efficiency being claimed for each by its originator. Sprockets typically do not have a flange. Some sprockets used with timing belts possess flanges to keep the timing belt centered. Sprockets and chains are also used for power transmission in one shaft to another where slippage is not admissible, sprocket chains being used instead of belts or ropes and sprocket-wheels instead of pulleys. They can be run at high speed and some types of chain are so built as to be noiseless even at high speed.