From Glossary of Meteorology
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A measure of the quantity of dissolved salts in seawater.

It is formally defined as the total amount of dissolved solids in seawater in parts per thousand (0/00) by weight when all the carbonate has been converted to oxide, the bromide and iodide to chloride, and all organic matter is completely oxidized. These qualifications result from the chemical difficulty in drying the salts in seawater. In practice, salinity is not determined directly but is computed from chlorinity, electrical conductivity, refractive index, or some other property with a relationship to salinity that is well established. The relationship between chlorinity Cl and salinity S as set forth in Knudsen's tables is
In 1940, however, a better expression for the relationship between total dissolved salts Σ and chlorinity was found to be
In more recent times, with the advent of devices that measure continuous records of conductivity electronically (e.g., CTD or conductivity–temperature–depth profiler), a new "practical salinity scale" has been determined. It is defined in terms of its electrical conductivity relative to a prescribed standard and it is given the units psu, for "practical salinity units." For most purposes one can assume that the new unit, psu, and the older unit, 0/00, are synonymous.