Noun the salt waters that cover the greater part of the earth's surface a division of these waters, of considerable extent, more or less definitely marked off by land boundaries the North Sea one of the seven seas;The Caspian Sea is an endorheic basin (drainage basin without an outflow) It covers a total surface area of about 386,400 km 2 and is about 1,0 km long and 3 km wide The sea's surface is approximately 27 m below sea level and has a volume of 78,0 km 3 The Caspian Sea accounts for between 4044% of the world's lacustrine watersSea definition 1 the salty water that covers a large part of the surface of the earth, or a large area of salty Learn more See The Difference Try Differentiating Seals From Sea Lions Cgtn Sea and sea