Salinity is the measure of the amount of dissolved salts in water. It is usually expressed in parts per thousand (ppt) or percent (%). 

Freshwater from rivers has a salinity of 0.5 ppt or less. Within the estuary, salinities are referred to as oligohaline (0.5-5.0 ppt), mesohaline (5.0-18.0 ppt), or polyhaline (18.0-30.0 ppt). Near the connection to the open ocean, estuarine water may be euhaline, where salinity is equivalent to that of the ocean at greater than 30.0 ppt.  

Salinity varies from place to place in the oceans, but the relative proportions of the major dissolved constituents remain virtually constant. Although there are smaller amounts of other ions in seawater (e.g., K+, Mg2+, SO42-), sodium (Na+) and chloride (Cl-) ions account for about 91% of all seawater ions. Fresh water has a much lower content of salt ions.