Salinity is how much salt is in water — most often sea water, or a lake. Scientists measure it; geography teachers talk about it. The Dead Sea has very high salinity; rain water has almost none. Though actually, the tidy line “containing salt” belongs more neatly to saline, the adjective. Salinity is the noun: not “it is salty”, but “how salty it is.” People sometimes stretch it to soil or food, but the home of the word is water.