Meaning
ヨウサイ (ようさい, Yōsai) is Water spinach, a Japanese food (leafy greens, plants, vegetables).
What is Water spinach?
Yōsai (蕹菜 or 甕菜, Ipomoea aquatica ), in English known as "water spinach", has countless local names in Southeast Asia. While yōsai is cultivated for its edible stems, it is closely related to sweet potatos, which also belong to the morning glory plant family. It is very popular in stir-fry dishes. The term "空芯菜[くうしんっさい]" also refers to water spinach.