Mochiko is one of those words that doesn't fare well in translation. It means “mochi flour” in Japanese, a definition helpful only to those already familiar with mochi, a type of soft and chewy rice cake. To better describe mochiko in English, most manufacturers call it sweet rice flour or glutinous rice flour instead.
Unfortunately, these names only cause confusion. Mochiko isn't sweet, nor does it contain a speck of gluten. It's entirely unlike traditional rice flour because it's made from sticky rice—the sort you have with slices of mango after a Thai meal. According to Shizuo Tsuji, the author of Japanese Cooking: A Simple Art, mochiko undergoes a unique milling process in which the sticky rice is first cooked, then dried and ground into a powder.
Once rehydrated, mochiko's chewy, sticky properties return. This glutinous behavior makes it an ideal ingredient for gluten-free baking.