Confectioners' sugar - you probably use it regularly when making icings or candy, but what is it exactly? It's granulated sugar that's been ground 10 times (that's why it's also called 10X) and then mixed with cornstarch, which helps prevent clumping. It's used in icings and candy because it dissolves easily.
You can make your own by grinding 1 cup of granulated sugar with 1 teaspoon of cornstarch in a blender for 3 minutes. Don't try doing this with a food processor - it won't work.