noun
brackish water area; estuarine waters
Technical term used in geography, ecology, and environmental science to refer to the zone where fresh water and seawater mix, such as an estuary.
汽水域には独特の生態系が形成される。
A unique ecosystem forms in brackish water areas.
海水 means 'seawater', while 汽水域 refers to the transitional zone where seawater and freshwater mix.
Compound of 汽 (ki, 'vapor' or 'steam', here referring to brackishness) + 水 (sui, 'water') + 域 (iki, 'area' or 'zone'). The term is a modern technical coinage.