training wheel

noun3

1. •
   [in the plural] (US, Canada) One of a pair of small wheels, each attached to one side of a bicycle's rear wheel to provide support for new riders.
2. •
   (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.
3. •
   (no longer used) Synonym of trailing wheel (“on a steam locomotive, an unpowered wheel or axle located behind the driving wheels”).