Figuring out which is which has driven me absolutely nuts over the years. So, what’s the answer?

There isn’t one. While most people can agree on what a glaze is, frosting and icing are essentially interchangeable even within the profession. But, since this is my blog and I like to make these things official, I’m going to give general definitions for all of these.

Glaze: A very thin topper. Can be either sweet, or savory–such as egg white used on breads and rolls to add gloss and crunch. As I’m allergic to egg that probably won’t come up much. However, sweet glazes almost certainly well. They’re typically somewhat syrupy in consistency. Consider the white, sugary glaze on some donuts and twisty pastries.
Icing: Midway between the two, not as thick as frosting. Will thicken and became somewhat hard. Like the chocolate used on donuts.

Frosting: Thick and creamy. The classic frosting. Remains fluffy. Prevalent on cakes and cupcakes.

There are, of course, umpteen more uses for, and styles of, frostings. These are just to give you a general idea of what sort of confection we’re dealing with here. They will be used in the title, if nowhere else.