training wheel

Short definition from Wiktionary

Noun

1. 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. 2
   Synonym of trailing wheel (“on a steam locomotive, an unpowered wheel or axle located behind the driving wheels”).
3. 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.