Longman Dictionary of Contemporary English
Wiktionary
n. (qualifier: Western US) Very finely grind sugar to which cornstarch is added to prevent lumping, mostly used in icings and confections to produce a smooth texture.
WordNet
n. sugar granulated into a fine powder
Wikipedia
Powdered sugar, also called confectioner's sugar or icing sugar, is a finely ground sugar made by milling normal granulated sugar into a powdered state. It usually contains a small amount of anti-caking agent—usually cornstarch in North America or tricalcium phosphate in other regions—to prevent clumping and improve flow. Although most often produced in a factory, it can also be made by putting normal sugar in a coffee grinder, or crushing it by hand in a mortar and pestle.
In industrial food production, it is used where a quick dissolving sugar is required. Domestically, it is principally used to make icing or frosting and other cake decorations. It is often lightly dusted onto baked goods to add a light sweetness and subtle decoration.
Powdered sugar is available in different degrees of fineness, most commonly XXX, XXXX, and 10X, with more Xs indicating finer grains. With finer particles more moisture is absorbed, which results in caking. An anti-caking agent is generally added during grinding, typically corn starch, or tricalcium phosphate, at 3% to 5% concentration, to absorb moisture and improve flow by reducing contact between sugar crystals.
Caster sugar (also referred to as superfine or baker's sugar) has a larger particle size than powdered sugar, approximately half that of granulated sugar.
Snow powder (or snow sugar) is a non-melting form of icing sugar usually consisting of dextrose, starch and anti-binding agents, useful for retaining its structure when dusted onto cakes or pastries that require refrigeration. It is mostly used for decorative purposes.
Usage examples of "powdered sugar".
O sacred butter cream, O clear to slightly cloudy Sunday afternoon dusted with powdered sugar!
Here is a formula for a bait-killer: 1 cup borax (another formulation of boric acid, available in the laundry section of grocery stores), 1/2 cup flour, 14 cup powdered sugar, 1/2 cup cornmeal.
But neither on the left where fat-saturated odors are meant to lure, nor on the right where powdered sugar is not being spared, are so many customers jostling as around the soon empty cardboard box.
Hesitantly Briaune brought a washcloth to the table to clean up the powdered sugar that she had spilled.
His face, his hands, his hair and the front of his clothes were all covered with a thorough dusting of powdered sugar.
He was idly licking the powdered sugar off his fingers as he spoke.
It just fractured it and rolled off, leaving behind a star-shaped cicatrice that looked like it was made of powdered sugar.
Ryan went for a mix of delicious sorbets coconut, banana, apple and cinnamon, decorated with powdered sugar, whipped cream and blueberries.
As he walked he remembered the taste of apple pie, and powdered sugar, and the sweet honey-cakes his father had made after they found the hive in the woods.