Definition of derailleur noun from the Oxford Advanced Learner's Dictionary

 

derailleur

 noun
noun
BrE BrE//dɪˈreɪljə(r)//
 
; NAmE NAmE//dɪˈreɪljər//
 
(specialist) Cycling
 
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a type of gear on a bicycle that works by lifting the chain from one gear wheel to another larger or smaller one See related entries: Cycling Word Origin 1930s: from French, from dérailler ‘derail’.
See the Oxford Advanced American Dictionary entry: derailleur