Basically a porridge (called kayu, more politely okayu, or in the Kansai familiarly "okayusan") made with rice and water flavored with some salt. Cooked with a little bit more water than usual. Although some toppings as an umeboshi, or furikake, may be used, these should be sober and the rice should not be cooked with ingredients because then it would become zosui.
Often eaten for breakfast instead of an ordinary bowl of rice, especially by the elderly or people who are ill. Soft and easily digestible. Also regularly served as part of the Japanese breakfast in hotels and ryokan.
Nanakusa-gayu or "seven herb porridge" is a dish traditionally eaten on January 7. Small amounts of seven different herbs are added to the porridge. This custom is believed to invite good luck and longevity in the new year.
In supermarkets, okayu is sold "ready to eat" in retort pouches.