noun
Water with salinity between fresh and seawater, typically found in estuaries.
この河口には半塩水の湿地が広がっている。
Brackish water wetlands spread across this river mouth.
淡水 means fresh water, with very low salinity.
海水 means seawater, with high salinity.
Compound of 半 (half) + 塩 (salt) + 水 (water), literally 'half-salt water'.