A popular Japanese rice balls formed into triangular or cylindrical shapes are called...
Traditionally, an onigiri is made from white rice and filled with pickled ume, salted salmon, katsuobushi, kombu, tarako, or any other salty or sour ingredient as a natural preservative. Most Japanese convenience stores stock their onigiri with various fillings and flavors. There are even specialized shops which only sell onigiri to take out.