sea

A sea is a large body of saltwater that is smaller than an ocean. Seas are usually partially enclosed by land and connected to one or more oceans. The term is used for many geographic features and for historical or cultural regions; seas can be open or enclosed, coastal or marginal.

Most seas are marginal seas formed by tectonics, sea-floor spreading, or coastal erosion. Notable examples include the Mediterranean Sea, Baltic Sea, Caribbean Sea, and South China Sea. Some bodies of water called seas are actually enclosed basins or lakes, such as the Caspian Sea, though common usage often treats them as seas.

Seas contain seawater with salinity around 3.5 percent, though salinity varies with latitude, altitude, and freshwater input. They are shaped by tides, winds, currents, evaporation, and river inflows, leading to seasonal and regional temperature and density differences. Coastal seas may be nutrient-rich and support substantial biological productivity.

Sea ecosystems host a wide range of organisms, from microscopic plankton to marine mammals, and include habitats such as coral reefs, seagrass beds, and kelp forests. Humans rely on seas for food, transportation, energy resources, and recreation, with fishing, shipping, oil and gas extraction, and tourism being major activities.

Environmental pressures include overfishing, plastic and chemical pollution, invasive species, habitat loss, and climate-change-related impacts such as warming, acidification, and sea-level rise. International law of the sea provides frameworks for navigation, territorial rights, and exclusive economic zones, while conservation efforts seek sustainable fisheries and protection of critical habitats.