a gear-shifting mechanism on a bicycle for controlling its speed
the sprocket chain from one to another of a set of different-sized sprocket wheels
Webster’s New World College Dictionary, 4th Edition. Copyright © 2010 by
Houghton Mifflin Harcourt. All rights reserved.
Word origin of 'derailleur'Fr dérailleur, derailer, with reference to the “derailing” of the chain from the sprocket