training wheel

(military, historical) A wheel forming part of the machinery of a cannon or a similar firearm which is turned to train or aim the weapon at a target.

(rail transport, archaic) Synonym of trailing wheel (“on a steam locomotive, an unpowered wheel or axle located behind the driving wheels”)

(chiefly Canada, US and in the plural, cycling) One of a pair of small wheels, each attached to one side of a bicycle's rear wheel to provide support for new riders.