Derailleur (noun)

1. A mechanism on a bicycle that allows the rider to change gears by moving the chain from one sprocket to another.
2. A similar mechanism on a train that diverts the wheels of a locomotive or a carriage from the main track to a secondary one.

Origin:

Early 20th century: french, from dérailler "to go off the rails".

Examples:

1. The derailleur on my bike needs to be adjusted because the gears are slipping.
2. The derailleur on a racing bike can have up to 12 gears to give the rider a wide range of gears to choose from.
3. The derailleur on the train was damaged in the crash, causing the train to jump off the tracks.
4. The derailleur is a crucial component of the bike's transmission system.
5. The derailleur system allows the bike to maintain an efficient and smooth gear change while riding.