Salinity refers to the concentration of dissolved ions in water, measured in parts per thousand (‰). Average ocean salinity is 35‰ but it varies depending on evaporation, precipitation, and water sources. Salinity is higher in enclosed seas like the Red Sea (40‰) and Mediterranean Sea (38‰) due to limited freshwater input and high evaporation. It is lower in semi-enclosed seas connected to oceans by narrow straits, like the Black Sea (18‰) and Baltic Sea (8‰), which receive more freshwater from rivers. Temperature, pressure, and density can also impact ocean salinity.