n. — Food
icing sugar.
Type: 2. Preservation — Confectioners sugar is another name for what is more commonly known as icing sugar or powdered sugar. Icing sugar is not Canadian, despite earlier claims (see entry), while confectioners' sugar, now often without the apostrophe, is a British preservation. Chart 1 shows that today the term is most common in Canada, followed by the UK. Chart 2 shows that another variant, powdered sugar, is common in Commonwealth countries, but of limited use in the US.
See also: berry sugar icing sugar icing ((3))
- Confectioners sugar (icing sugar) is finely granulated sugar with added cornstarch (see Canadian Sugar Institute reference).
- 1888  Grate rind from one orange, add a speck of tartaric acid, 2 large tablespoonfuls of powdered or confectioners' sugar, and enough orange juice to mix into a smooth paste. 
- 1891  Shell the peanuts, remove the skin, and break into small pieces, or not, as preferred. Take two cups of the confectioners' sugar and one cup of the peanuts. Put the sugar in a saucepan, and as soon as dissolved throw into it the nuts, stirring rapidly. 
- 1922  Stir in confectioner's sugar until of right consistency to spread. 
- 1942  Cover with whipped cream sweetened with confectioners' sugar. 
- 1963  Bake in moderate oven (350° (176) F.) about 30 minutes, or until done. Frost with confectioners sugar frosting. Yield: one stollen. 
- 1982  In small bowl, combine cream, confectioners' sugar and vanilla. 
- 1993  Line a baking sheet with waxed paper and dust generously with Confectioner's sugar. 
- 2005  4 tablespoons (60 mL) granulated sugar substitute or confectioners sugar, divided [...] 
- 2014  Icing Sugar: Also known as confectioner's sugar or powdered sugar, it's actually a combination of very finely ground granulated sugar with the addition of cornstarch, which acts as an anti-clumping agent (ie. keeps it from turning into a solid lump of sugar when humidity is present). Dissolves very quickly, but can't be used as a straight-up replacement for other sugars due to the additional cornstarch. 
- 2016  The one store-bought "food" I came to crave most was canned rainbow chip frosting: I didn't taste it until I was about 10 years old and at a friend's house, because at home, frosting was just a mix of confectioners' sugar, milk, vanilla extract and a little melted butter. 
Images:
Chart 1: Internet Domain Search, 4 Jul. 2014
Chart 2: Internet Domain Search, 26 Nov. 2015