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
Images:

Image 1: A cake covered with icing (Source: Wikimedia Commons. Photo: Scheinwerfermann)
Chart 1: Internet Domain Search, 21 Mar. 2016