training wheel

Definitions

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.
   historical
2. Synonym of trailing wheel (“on a steam locomotive, an unpowered wheel or axle located behind the driving wheels”).
   archaic
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.
   Canada, US, in-plural

Equivalents

CEFR level