caster sugar 
(also castor sugar)
noun [mass noun] British
Finely granulated white or pale golden sugar.In a bowl mix 200g medium desiccated coconut, 50g golden caster sugar and 1 egg white....
- When the colour is exactly right, I will stir in exactly one rounded teaspoonful of golden caster sugar.
- Put the cream cheese, mascarpone, caster sugar, eggs and the extra yolk in the basin of a food mixer (you will need the beater attachment).
Origin
Mid 19th century: so named because it was suitable for use in a castor (see castor1 (sense 2)).