/ Dictionary / Index D derailleur: Meaning and Definition of Find definitions for: de•rail•leur Pronunciation: (di-rā'lur), [key] — n. a gear-shifting mechanism on a bicycle that shifts the drive chain from one sprocket wheel to another. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. derail derailment Related Content Daily Word Quiz: reclusive Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language