1Foam or a bubble in a liquid; a water bubble or froth.
波が岩に当たって水泡が立った。
Waves hit the rocks and foam bubbled up.
シャンプーを手にとると水泡がたくさんできた。
When I took the shampoo in my hands, a lot of lather formed.
ビーカーの中の液体に小さな水泡が見える。
Small bubbles are visible in the liquid in the beaker.
2A figurative sense meaning that efforts or plans come to nothing (as in the phrase 水泡に帰す).
長年の研究が資金不足で水泡に帰してしまった。
Years of research came to nothing due to lack of funding.
彼の努力は一つのミスで水泡に帰した。
His efforts were undone by one mistake.