Salinity is the mass of salts contained in 1 kg of seawater. Currently this is obtained by measuring the conductivity and is expressed in psu (practical salinity units), approximately equivalent to 1 mg/g of salts. The salinity of seawater is on average 35 psu, i.e. about 35 g/kg.
Salinity is one of the physical-chemical properties of water.
Salinity measures the concentration of dissolved salts in water (sodium chloride, magnesium chloride, magnesium sulphate etc.) using its electric conductivity.
This salinity has no units, but it is still often expressed in grams of salt per kilogram of water (g/kg), in parts per thousand or in practical salinity units (psu).
Technically, salinity is defined as the mass (in grams) of substances dissolved in one kilogram of seawater when the bromide and iodide ions are replaced by their equivalent in chloride, the carbonates converted into oxides and all of the organic matter is oxidised.
As this value is difficult to obtain, scientists rely on the fact that the proportionality of various salts is constant to define other quantities related to salinity: chlorinity and chlorosity.
As a comparison, fresh water has a salinity from 0 to 0.5 and seawater (euhaline water) has an average salinity of 35.
Ocean salinity measuring campaign. © Capitaine Robert A. Pawlowski / NOAA