Granulated sugar that has been crushed into a fine powder with about 3% cornstarch added to prevent clumping. Powdered sugar labeled XXXX is slightly finer than that labeled XXX but they can be used interchangeably. 1 pound = 4 cups sifted.
Because it dissolves so quickly it is often used for icings and frostings or dusted over desserts.
1 3/4 cup powdered sugar = 1 cup granulated sugar
1 cup granulated sugar + 1 tsp. cornstarch processed in a blender (NOT a food processor) = 1 cup powdered sugar