The Aral Sea (Kazakh: Арал Теңізі Aral Teñizi; Uzbek: Orol Dengizi; Russian: Аральскοе Мοре Aral'skoye More; Tajik: Баҳри Арал Bakhri Aral; Persian: ‎دریاچه خوارزم Daryâche-ye Khârazm) was a lake that lay between Kazakhstan (Aktobe and Kyzylorda provinces) in the north and Karakalpakstan, an autonomous region of Uzbekistan, in the south. The name roughly translates as "Sea of Islands", referring to more than 1,534 islands that once dotted its waters; in Old Turkic "aral" is island and thicket.

Formerly one of the four largest lakes in the world with an area of 68,000 square kilometres (26,300 sq mi), the Aral Sea has been steadily shrinking since the 1960s after the rivers that fed it were diverted by Soviet irrigation projects. By 2007, it had declined to 10% of its original size, splitting into four lakes – the North Aral Sea, the eastern and western basins of the once far larger South Aral Sea and one smaller lake between North and South Aral Seas. By 2009, the southeastern lake had disappeared and the southwestern lake retreated to a thin strip at the extreme west of the former southern sea. The maximum depth of the North Aral Sea is 42 m (138 ft) (as of 2008).

A sea generally refers to a large body of salt water, but the term is used in other contexts as well. Most commonly, it means a large expanse of saline water connected with an ocean, and is commonly used as a synonym for ocean. It is also used sometimes to describe a large saline lake that lacks a natural outlet, such as the Caspian Sea.

Arctic (belonging to the Arctic Ocean) and Antarctic (Southern Ocean) seas, as well as some other seas freeze in winter. This occurs below the freezing point of pure water, at about -1.8 °C (28.8 °F). Frozen salt water becomes sea ice.

Humans navigated seas from antiquity. Ancient Egyptians and Phoenicians navigated the Mediterranean Sea and the Red Sea. Hannu was the first see explorer of whom there is any information. He sailed along the Red Sea and further to the Arabian Peninsula and the African Coast around 2750 BC. In the 1st millennium BC, Phoenicians and Greeks established colonies all over the Mediterranean, including its outlets like the Black Sea. The seas along the eastern and the southern Asian coast were used by Arabs and Chinese for navigation, and the North Sea and the Baltic Sea were known to Europeans in Roman times. Other seas were not used for navigation in the antiquity and were actually discovered.