derailleur

noun

1. a gear-shifting mechanism on a bicycle that shifts the drive chain from one sprocket wheel to another.

noun

1. a mechanism for changing gear on bicycles, consisting of a device that lifts the driving chain from one sprocket wheel to another of different size

n.

type of bicycle gear mechanism, 1930, from French dérailleur (1927), from dérailler “to go off the rails” (see derail).