derailleur/ˌdəˈɹeɪl.jə/EtymologyBorrowed from French dérailleur. Doublet of derailer.nounThe mechanism on a bicycle used to move the chain from one sprocket (gear) to another.“The common rear derailleur designs all operate on the same principle. A moveable cage receives the chain returning from the chainwheel and directs it onto one sprocket of the cluster.”The entire gearset on a bicycle with such a mechanism.