Saltwater, or salt water, is a geological term that refers to naturally occurring solutions containing large concentrations of dissolved, inorganic ions. In addition, this term is often used as an adjective in biology, usually to refer to marine organisms, as in saltwater fish.

Saltwater most commonly refers to oceanic waters, in which the total concentration of ionic solutes is typically about 35 grams per liter (also expressed as 3.5%, or 35 parts per thousand). As a result of these large concentrations of dissolved ions, the density of saltwater (1.028 g/L at 4° C) is slightly greater than that of freshwater (1.00 g/L). Therefore, freshwater floats above saltwater in poorly mixed situations where the two types meet, as in estuaries and some underground reservoirs.

The ions with the largest concentrations in marine waters are sodium, chloride, sulfate, magnesium, calcium, potassium, and carbonate. In oceanic waters, sodium and chloride are the most important ions...