n. — Food
a mixture usually based on sugar and butter used to decorate or cover cakes and other baked goods (see Image 1).
Type: 2. Preservation — Icing contains sugar or another sweetener mixed with a thickener such as butter or egg whites, among other ingredients. The term is a preservation from British English, where it appears as early as the 17th century (see OED-3, s.v. "icing" n. (1a)). Chart 1 demonstrates the term's use in former British colonies. The term is important in the Canadian context as it contrasts with the most frequent US term, "frosting". Boberg (2005: 58) confirms in his questionnaire survey that the "majority Canadian term" is icing, which "also dominates the Midland and Southern United States". However, he notes that "frosting" is the "majority term in the remainder of the United States" and says that "frosting" is "rare in most of Canada".
See also: icing sugar confectioners sugar
- 1845  TO MAKE ICING FOR CAKES. Take a pound of triple refined sugar, pound and sift it through a silk searce [a fine sieve], put it in a bowl, add the whites of three eggs, and the juice of half a small lemon; beat it with a wooden spoon continually for an hour [...] 
- 1886  WHITE CAKE. [...] for icing and filling, white of three eggs and three cups of powdered sugar, with juice and grated peel of a lemon. 
- 1910  Almond paste, ready prepared for Christmas cake icing, is available at 40c per pound. 
- 1940  "You aren't a very striking advertisement for love in a cottage. I mean you were so in love with Bill I used to envy you. No [sic] all the icing seems to have come off the wedding cake or has it?" 
- 1961  Combine the egg whites, salt, vanilla and sufficient icing sugar to make an icing of spreading consistency - about 2½ cups. 
- 1989  Because this cake is very moist and will keep beautifully, it needs a good rich icing that will keep well and not dry out. 
- 2001  To make icing, cream butter and icing sugar. Add vanilla and milk and beat until smooth and creamy. Spread icing over the top of the cake. 
- 2016  Dinner at 10:30 a.m. or supper at 4 p.m. might consist of the following: brown or white beans boiled with ham or bacon, boiled potatoes, roast beef and gravy, boiled rice and raisins (called spotted dog), plain cake with cornstarch icing, blueberry, apple or raisin pie, hot sourdough biscuits, coffee or tea. 
Images:
Image 1: A cake covered with icing (Source: Wikimedia Commons. Photo: Scheinwerfermann)
Chart 1: Internet Domain Search, 21 Mar. 2016