noun
1.
a. : a horizontal wheel or segment of a wheel that consists of two parts rotating on each other about the kingbolt above the fore axle of a carriage or wagon and beneath the body and that forms an extended support to prevent tipping
b. : a coupling in the form of two disks rotating on each other for attaching a vehicle body to the front axle so as to support it in turning
c. : a similar coupling between tractor and trailer of a semitrailer
2.
a. : a spare wheel
b. : a light wheel trailed behind an automobile for measuring speed or distance
3. : one that is superfluous or burdensome
committees without authority to act are fifth wheels — F.L.Allen