Good question :) I've always called it "icing," as I dislike the word "frosting" (for no good reason....just bugs me...heh). I see them as essentially the same thing.....that is, of course, if the stuff betwixt the layers was the same stuff on top - or was it "filling" ? ;)
FWIW, a quick web search at Yahoo Dictionary for "frosting" turned up this gem..." frost·ing
- Icing, as on a cake.
Although both frosting
are widespread, people in New England, the Upper Midwest, and the Western U.S. tend to put frosting
on cake. In Pennsylvania, New Jersey, the Lower Midwest, and all of the South, the preferred term is icing.
There is some overlap, especially in upstate New York, Michigan, and California, but the regions in which the two words predominate are surprisingly distinct. A few people in the South call it by a third name, filling,
even when it goes on top. "
So, I guess it depends on where you're from ? Appropriately enough, I was born and raised in PA, so perhaps that explains my preference for the term "icing" !!!