Water in the gaseous state (water vapor or steam); moisture present in the air which can condense into vapor or droplets.
水蒸気は冷えると小さな水滴になり、霧や雲を作る。
When water vapor cools, it becomes tiny droplets and forms fog or clouds.
ボイラーは高温の水蒸気を発生させる設備だ。
A boiler is equipment that generates high-temperature steam.
湿度が高い日は空気中の水蒸気が多い。
On humid days there is more water vapor in the air.