Smaller than an ocean and often partly enclosed by land, a sea is a large body of saltwater. Examples include the Mediterranean, the Caribbean, and the South China Sea. Some seas, such as the Caspian, are landlocked.

Seas are influenced by both oceanic and terrestrial processes. They receive freshwater and sediment from rivers and experience tides and currents connected to the wider ocean. Many have been vital corridors of trade and cultural exchange for millennia.

Human pressures on seas include overfishing, pollution, and coastal development. Semi-enclosed seas are especially vulnerable because water exchange with the open ocean is limited. International agreements attempt to manage shared seas, though enforcement varies. The distinction between sea and ocean is partly conventional yet remains useful in geographic description.

• Large body of saltwater, smaller than an ocean
• Often partly enclosed by land
• Influenced by rivers and tides
• Historically important for trade and culture