What is the name of the mechanism that moves the chain between gears on a bicycle? It is called a derailleur.
A derailleur shifts the moving chain from one sprocket or chainring to another, changing the bicycle's gear ratio. Most multi-speed bicycles use a rear derailleur, while bicycles with multiple front chainrings may also have a front derailleur.
The rider operates a shifter, which pulls or releases a cable—or commands an electronic actuator. That movement positions a chain guide beside the selected sprocket. The chain then climbs onto the new sprocket while the bicycle is being pedalled. A rear derailleur also takes up slack as the chain moves between differently sized sprockets.
A shifter is not the mechanism that physically guides the chain; it is the control. A chainring is a toothed front ring, and a cassette is the cluster of rear sprockets. The word derailleur comes from French and literally relates to derailing from a track.