Salinity
Salinity is the concentration of dissolved salts in water, averaging about 35 grams per kilogram in the open ocean.
Definition
Salinity is how salty water is, expressed as the total dissolved salts it carries, and ocean water is salty because rivers have carried dissolved minerals into it for billions of years while evaporation removes only the water, leaving the salt behind, mostly sodium chloride along with magnesium, sulfate, calcium, and potassium. Average open-ocean salinity is about $35$ grams per kilogram, higher where evaporation is strong, such as the subtropics or the Red Sea, and lower where rain, rivers, or melting ice dilute it, and the relative proportions of major ions stay remarkably constant everywhere because ocean mixing, taking about a thousand years, is fast compared to the millions of years sodium and chloride spend dissolved before removal in evaporites, sea spray, and clay formation. Salinity combines with temperature to set seawater density, which is the engine behind deep ocean circulation.
Example
Boiling away a liter of seawater leaves about $35$ grams of salt behind, roughly two tablespoons, while Pamlico Sound behind the Outer Banks runs brackish at around $15$ to $20$ ppt because rivers dilute it, and the Dead Sea, at over $300$ ppt, is so salty that swimmers float without effort. The Atlantic is saltier than the Pacific, about $35.5$ versus $34.5$, because trade winds carry evaporated Atlantic moisture across Central America, which is one reason deep water forms in the North Atlantic rather than the North Pacific, and satellites such as SMOS and Aquarius now map ocean surface salinity to trace the water cycle over the sea.
Key Insight
Rivers taste fresh, yet they are exactly what made the ocean salty over immense stretches of time, each one delivering a tiny trickle of dissolved minerals that the ocean has been collecting ever since, and freezing seawater leaves its salt behind entirely, so sea ice is nearly fresh while the extra-salty water it rejects sinks and helps drive the deep currents. Because salty regions are growing saltier and fresh regions fresher, ocean salinity patterns now serve as a rain gauge for the planet, recording an intensifying water cycle that is difficult to measure directly over open water.