A sea is a large body of salt water. There are particular seas and the sea. The sea commonly refers to the ocean, the interconnected body of seawaters that spans most of Earth. Particular seas are either marginal seas, second-order sections of the oceanic sea (e.g. the Mediterranean Sea), or certain large, nearly landlocked bodies of water.

The salinity of water bodies varies widely, being lower near the surface and the mouths of large rivers and higher in the depths of the ocean; however, the relative proportions of dissolved salts vary little across the oceans. The most abundant solid dissolved in seawater is sodium chloride. The water also contains salts of magnesium, calcium, potassium, and mercury, among other elements, some in minute concentrations. A wide variety of organisms, including bacteria, protists, algae, plants, fungi, and animals live in various marine habitats and ecosystems throughout the seas. These range vertically from the sunlit surface and shoreline to the great depths and pressures of the cold, dark abyssal zone, and in latitude from the cold waters under polar ice caps to the warm waters of coral reefs in tropical regions. Many of the major groups of organisms evolved in the sea and life may have started there.

The ocean moderates Earth's climate and has important roles in the water, carbon, and nitrogen cycles. The surface of water interacts with the atmosphere, exchanging properties such as particles and temperature, as well as currents. Surface currents are the water currents that are produced by the atmosphere's currents and its winds blowing over the surface of the water, producing wind waves, setting up through drag slow but stable circulations of water, as in the case of the ocean sustaining deep-sea ocean currents. Deep-sea currents, known together as the global conveyor belt, carry cold water from near the poles to every ocean and significantly influence Earth's climate. Tides, the generally twice-daily rise and fall of sea levels, are caused by Earth's rotation and the gravitational effects of the Moon and, to a lesser extent, of the Sun. Tides may have a very high range in bays or estuaries. Submarine earthquakes arising from tectonic plate movements under the oceans can lead to destructive tsunamis, as can volcanoes, huge landslides, or the impact of large meteorites.

The seas have been an integral element for humans throughout history and culture. Humans harnessing and studying the seas have been recorded since ancient times and evidenced well into prehistory, while its modern scientific study is called oceanography and maritime space is governed by the law of the sea, with admiralty law regulating human interactions at sea. The seas provide substantial supplies of food for humans, mainly fish, but also shellfish, mammals and seaweed, whether caught by fishermen or farmed underwater. Other human uses of the seas include trade, travel, mineral extraction, power generation, warfare, and leisure activities such as swimming, sailing, and scuba diving. Many of these activities create marine pollution.

The sea is the interconnected system of all the Earth's oceanic waters, including the Atlantic, Pacific, Indian, Southern and Arctic Oceans. However, the word "sea" can also be used for many specific, much smaller bodies of seawater, such as the North Sea or the Red Sea. There is no sharp distinction between seas and oceans, though generally seas are smaller, and are often partly (as marginal seas or particularly as a mediterranean sea) or wholly (as inland seas) enclosed by land. However, an exception to this is the Sargasso Sea which has no coastline and lies within a circular current, the North Atlantic Gyre. Seas are generally larger than lakes and contain salt water, but the Sea of Galilee is a freshwater lake. The United Nations Convention on the Law of the Sea states that all of the ocean is "sea".

The law of the sea has at its centre the definition of the boundaries of the ocean, clarifying its application in marginal seas. But what bodies of water other than the sea the law applies to is being crucially negotiated in the case of the Caspian Sea and its status as "sea", basically revolving around the issue of the Caspian Sea about either being factually an oceanic sea or only a saline body of water and therefore solely a sea in the sense of the common use of the word, like all other saltwater lakes called sea.^{[citation needed]}

Earth is the only known planet with seas of liquid water on its surface, although Mars possesses ice caps and similar planets in other solar systems may have oceans. Earth's 1,335,000,000 cubic kilometers (320,000,000 cu mi) of sea contain about 97.2 percent of its known water and covers approximately 71 percent of its surface. Another 2.15% of Earth's water is frozen, found in the sea ice covering the Arctic Ocean, the ice cap covering Antarctica and its adjacent seas, and various glaciers and surface deposits around the world. The remainder (about 0.65% of the whole) form underground reservoirs or various stages of the water cycle, containing the freshwater encountered and used by most terrestrial life: vapor in the air, the clouds it slowly forms, the rain falling from them, and the lakes and rivers spontaneously formed as its waters flow again and again to the sea.

The scientific study of water and Earth's water cycle is hydrology; hydrodynamics studies the physics of water in motion. The more recent study of the sea in particular is oceanography. This began as the study of the shape of the ocean's currents but has since expanded into a large and multidisciplinary field: it examines the properties of seawater; studies waves, tides, and currents; charts coastlines and maps the seabeds; and studies marine life. The subfield dealing with the sea's motion, its forces, and the forces acting upon it is known as physical oceanography. Marine biology (biological oceanography) studies the plants, animals, and other organisms inhabiting marine ecosystems. Both are informed by chemical oceanography, which studies the behavior of elements and molecules within the oceans: particularly, at the moment, the ocean's role in the carbon cycle and carbon dioxide's role in the increasing acidification of seawater. Marine and maritime geography charts the shape and shaping of the sea, while marine geology (geological oceanography) has provided evidence of continental drift and the composition and structure of the Earth, clarified the process of sedimentation, and assisted the study of volcanism and earthquakes.