A sea is a large body of salt water that is partly or fully enclosed by land, or that forms a distinct region of the global ocean. Examples include the Mediterranean Sea, the South China Sea, and the Caribbean Sea. The distinction between “sea” and “ocean” is largely conventional rather than strictly scientific.

Seas often have unique circulation, chemistry, and ecosystems shaped by their restricted exchange with the open ocean and by strong terrestrial influences. Semi-enclosed seas can experience amplified responses to climate change, nutrient loading, and fishing pressure because of their limited flushing. Some seas, such as the Caspian, are entirely landlocked yet still saline.

Historically, seas have been the focus of navigation, trade, and naval power. Today they remain important for fisheries, hydrocarbon extraction, and regional climate. Scientific study of individual seas combines oceanography with coastal geology and meteorology to understand their particular dynamics.

The Mediterranean Sea, for example, has a residence time of about a century and a distinctive temperature and salinity signature.