Brackish refers to water that is saltier than fresh water, but not as salty as sea water. It may result from mixing of seawater with fresh water, as in estuaries, or it may occur naturally, as in brackish fossil aquifers.
Technically, brackish water contains between 0.5 and 30 grams of salt per litre—more often expressed as 0.5 to 30 parts per thousand (ppt or o/oo). Thus, brackish covers a range of salinity regimes.
The term brackish water derives from the Low Saxon word brackwater, which is the water of a brack. A brack is a small lake created when a storm tide breaks a dike and floods land behind the dike.