Zōsui (雑炊) is a Japanese rice soup made from pre-cooked rice and water. Okayu, which is a similar dish to Zosui, is rice cooked to a watery consistency.
Zosui (which is also called ojiya) and Okayu, while similar, are two separate dishes. While the two dishes both contain rice, and both are traditionally eaten when people are feeling unwell, okayu is a kind of porridge (similar to oatmeal), while zosui is akin to a vegetable soup with lots of rice.
Leftover soup from nabe is often re-used for zosui. Instead of rice, udon and ramen noodles are recent alternatives.