Salinity
What is it? Salinity in saltwater is a measurement of salts dissolved in water that come from rocks and soil.
Why is it important? There are two factors that affect ocean water density. The most important one is temperature, and the second most important is salinity. Density affects currents and much more. Salinity measures can also tell us how much freshwater is mixing with sea water and indicate which species we should expect there.
What does it mean? The average salinity of the ocean is 35 ppt (parts per thousand). But, things like melting ice and rivers dilute salty water, so many coastal areas have lower readings. Our oceans vary in salinity too: Near the shores, rivers dilute the ocean to reduce salinity. In the arctic, melting icecaps also affect salinity.