training wheel

Meanings

1. 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.
2. Synonym of trailing wheel (“on a steam locomotive, an unpowered wheel or axle located behind the driving wheels”).
3. One of a pair of small wheels, each attached to one side of a bicycle's rear wheel to provide support for new riders.

Etymology

From training (“relating to the moving of a firearm laterally so that it points in a different direction; relating to imparting and acquiring skills”) + wheel.