"Basically water that is a mix of fresh and salt."
Technically that what sea water is too.
Rob.B brackish water is generally found a few miles up rivers that dump into a salt water body. Around the mouth or delta of a river your water in it is usually the same salt to water ratio that it is in the body that it is dumping into. As you travel up stream the salt saturation decreases, once it gets to a certain number it is called brackish, then after that you have fresh water.
This is very off topic, but I believe I read about some kind of micro organism one time they found in brackish water that had infected an killed a couple of swimers. They said brackish water was the only enviroment that is could survive in, but it rarly is the cause of human fatality. Anyone ever hear of this?