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A salt is the product (along with water) of a chemical reaction between a base and an acid. They are ionic compounds, usually soluble in water.

In common usage, the term "salt" refers to a specific salt, sodium chloride. This salt is mostly responsible for the salinity of the ocean waters and the extra-cellular fluid of many multi-cellular organisms.

Salt is essential for life. Humans are unusual amoungst primates both in secreting large amounts of salt during the process of sweating, and also not appearing to possess a "salt appetite".

Because of its importance for life, control over salt has often been used for social and political ends. The English word "salary" derives from the Latin word for "salt", which in the Roman Empire was sometimes used as a form of currency. The empire of Mali, in Africa, valued salt enough to buy it for its weight in gold; this trade led to the legends of the incredibly wealthy city of Timbuktu, and fueled inflation in Europe, which was exporting the salt. In later times, for instance during the British colonial period, salt production and transport was controlled in India as a means of generating enormous tax revenues.

Salt is commonly used as a flavour enhancer for food and has been identified as one the basic tastes. Ironically, given its history, this has resulted in large sections of the developed world ingesting salt massively in excess of the required intake, particularly in colder climates where the required intake is much lower. This is believed to cause elevated levels of blood pressure in some, which in turn in associated with increased risks of heart attack and stroke.

Salt is produced by evaporation of sea water or brine from other sources, such as brine wells and salt lakes, and from mining of rock salt. Long a scarce commodity, industrialised production has made salt plentiful. About 51% of world output is now used by northern countries to de-ice roads in winter.