Definition of "derailer"

Very few bicycles are of a different color or employ more gears with derailer sprockets and hand brakes.

2009, Louis G. Perez, “Society in the Twenty-First Century”, in The History of Japan (The Greenwood Histories of the Modern Nations), 2nd edition, Westport, Conn., London: Greenwood Press, page 202

Through sheer ingenuity and resourcefulness, these men handproduced parts such as derailers, stands, seat posts, cranks, fenders, and all manner of elements used in the production of a functional bicycle.

2009, Christine Elliott, David Jablonka, “Cycles Alex Singer”, in Custom Bicycles: A Passionate Pursuit, Images Publishing, page 72, column 2

Inside, two bearded men are working on a busted fuel pump, and another is repairing a bicycle derailer.

2016, Erik Reece, Utopia Drive: A Road Trip Through America’s Most Radical Idea, New York, N.Y.: Farrar, Straus and Giroux, page 170

Recent American attempts to further Anglicize the word to “derailer” have met with little success. In any case, no rails are involved.]

[2016, Tony Hadland, Hans-Erhard Lessing, Bicycle Design: An Illustrated History, MIT Press, page 246