Definition of "derailleur"

derailleur

/ˌdəˈɹeɪl.jə/

noun
plural derailleurs
1. The mechanism on a bicycle used to move the chain from one sprocket (gear) to another.
  Quotations
  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.
  1993, John Forester, “Adjusting Derailleurs”, in Effective Cycling, MIT Press, page 14
2. The entire gearset on a bicycle with such a mechanism.