Freewheel

[1] Usually refers to the action of a vehicle on a downgrade when the overdrive over-running clutch is slipping with a resultant loss of engine braking. This condition will occur only after the overdrive unit is engaged but before the balk ring has activated the planetary gearset.
[2] The removable cluster of gears at the hub of the rear wheel of a bicycle. Usually there are five or more gears. It also contains a ratcheting mechanism inside that allows the wheel to rotate forward while the pedals, chain, and gear sprockets remain still or move in reverse. The rear derailleur moves the chain from one gear to the other to change the gear ratio. The other end of the chain wraps around the chainwheel. The freewheel threads onto the hub while the newer style of gear cluster (cassette) slides onto the hub.