A small wheel attached to the bottom of furniture that allows it to roll, or a container with holes for sprinkling salt or sugar.
Possibly from 'cast' (to throw) or from a person's name. The wheel meaning emerged in the 18th century; the sprinkling container (like a caster sugar shaker) came later.
Caster wheels seem simple, but they're engineered so the wheel swivels and the axis is offset, which creates an invisible force called 'caster angle' that magically keeps rolling chairs from spinning wildly.
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