Onigiri is a ball of cooked Japanese rice. It sometimes goes by another name, musubi. It is the equivalent of a sandwich in Japan, made in family kitchens, and also available at corner stores and delis all over the country.
Onigiri or musubi have been consumed in Japan for over a thousand years as a convenient and portable food to take into battle, while working in the fields, heading to the office or school, or on a train ride or picnic. It is the food of both hard work and leisure.
To make onigiri, all you need is rice, and it needs to be japonica rice, otherwise it wont stick. To make the rice ball more interesting and nutritious, different ingredients are often added as a filling or are mixed in with the cooked rice, and a nori wrapper is often added as a neat way to hold it in your hand.
Interview about onigiri with the head chef of Waketokuyama (a two Michelin star restaurant in Tokyo)
Press release by Onigiri Society in Tokyo