1Water that has evaporated into a gaseous state; invisible to the eye, but becomes visible as droplets or steam when cooled.
鍋のふたを開けると、水蒸気がもうもうと立ち上った。
When I took the lid off the pot, steam billowed up.
空気中の水蒸気が冷やされて雲になる。
Water vapor in the air is cooled and turns into clouds.
窓ガラスが水蒸気で曇っている。
The window glass is fogged up with water vapor (condensation).