Castor sugar

Also spelled caster, castor sugar is a granulated sugar that has been ground more finely than regular table sugar but not as fine as powdered sugar. It is actually the British equivalent of American powdered sugar. Castor sugar is used when it is necessary to use a sugar that will dissolve very quickly. The name derives from its association with the tall cylindrical vessel with a perforated lid, known in England as a castor, often used to store and dispense it.