Rock sugar (bīngtáng, 冰糖) is large-crystal sugar used in Chinese cooking. It is typically made from sugarcane, has a pale yellow or white color, and provides a straightforward sweet flavor.
Contrast brown rock sugar (bīng piàntáng, 冰片糖) which contains more molasses and adds additional caramel flavors.
From a strict standpoint, rock sugar is the same as rock candy, but they are packaged differently and sold for very different purposes.