A pivoting roller attached to the bottom of furniture or trucks or portable machines to make them movable WordNet 3.0 & Open English WordNet

A caster is an undriven wheel that is designed to be attached to the bottom of a larger object to enable that object to be moved. Wikipedia

Type of wheel. Wikipedia Disambiguation

An undriven wheel Wikipedia Disambiguation

A wheeled assembly attached to a larger object at its base to facilitate rolling. A caster usually consists of a wheel (which may be plastic, a hard elastomer, or metal), an axle, a mounting provision (usually a stem, flange, or plate), and sometimes a swivel (which allows the caster to rotate for steering). Wiktionary

Wheeled assembly. Wiktionary (translation)