A starchy substance extracted from the root of the cassava plant. It's available in several forms including granules, flakes, pellets (called pearl tapioca) and flour or starch. The most widely available forms are tapioca flour (also called cassava flour) and pearl tapioca. The flour is used as a thickening agent for soups, fruit fillings, glazes, etc. , much like cornstarch. Pearl tapioca is used mainly to make pudding and comes in several sizes, regular or instant forms and in a variety of prepackaged flavors. Pearl tapioca is available in most supermarkets, whereas the other forms are more commonly found in health-food stores and Asian markets. If stored in a cool, dark place, all types of tapioca will keep indefinitely.