A sea is either a large expanse of saline water connecte with an ocean, or a large, usually saline, lake that lacks a natural outlet such as the Caspian Sea and the Dead Sea. Colloquially, the term is used synonymous with ocean. Large lakes, such as the Great Lakes, are sometimes referred to as inland seas. Marginal seas are those that have currents caused by ocean winds, and mediterranean seas are those in which currents are caused by differentials in salinity and temperature.