Answer
Castor or caster sugar is the name of a very fine sugar, so named because the grains are small enough to fit though a sugar "caster" or sprinkler. Because of its fineness, it dissolves more quickly than regular white sugar, and so is especially useful in meringues and cold liquids. It is not as fine as confectioner’s sugar, which has been crushed mechanically (and generally mixed with a little starch to keep it from clumping).