© Denzil Green
Icing sugar is called Confectioners' Sugar or Powdered Sugar in the United States.
It's a commercial product that is very hard to make at home. Sugar is pounded and ground by machines into a very fine white powder. Cornstarch is then added (up to 3% of the overall weight of the sugar) to stop the sugar from caking or clumping. In Australia, cornstarch is not added. Australia also has also flavoured icing sugars.
In Scandinavian countries, potato starch (up to 2%) is used in place of the cornstarch.
It is used primarily for making icings and frostings, and for dusting baked goods with. Only rarely does a recipe -- such as Nanaimo Bars -- call for it as an ingredient.
10x Powdered SugarYou may also see varieties labelled 4x and 10x; this is really fine, and really, really fine. The numbers refer to how many times the sugar has been ground. Though the 10x is slightly easier to whip, you can use either interchangeably.
In a recipe (but not for icing or frosting), you can substitute white sugar at the ratio of 1 cup (8oz / 225g) of white sugar for every 1 3/4 cups (12 oz / 350g) Icing Sugar.
4 oz = 2/3 cup = 115g
2 oz = 60g = 5 tablespoons
1 pound = 450g = 3 3/4 cups
1 oz Icing Sugar = 30g = 2 1/2 tablespoons