A sea is a large expanse of saline water connected with an ocean, or a large, usually saline, lake that lacks a natural outlet such as the Caspian Sea and the Dead Sea. The Sea of Galilee is a small freshwater lake with a natural outlet, but the term was applied to it anyway. The term is used colloquially as synonymous with ocean, as in the tropical sea or down to the sea shore, or even sea water referring to water of the ocean.
Many seas are marginal seas.
List of seas, divided by ocean
Lunar maria are vast basaltic plains on the Moon that were thought to be bodies of water by early astronomers, who referred to them as "seas".
Liquid water is thought to be present under the surface of several moons, most notably Europa.
Liquid hydrocarbons are thought to be present on the surface of Titan, though it may be more accurate to describe them as "lakes" rather than "seas". The distribution of these liquid regions will hopefully be better known after the arrival of the Cassini probe.
The term "sea" has also been used in quantum physics. Dirac sea is an interpretation of the negative energy states that comprises the vacuum.