The degree Celsius (°C) is a unit of temperature named after the Swedish astronomer Anders Celsius (1701–1744), who first proposed a similiar system in 1742. The Celsius temperature scale was designed so that the freezing point of water is 0 degrees, and the boiling point is 100 degrees at standard atmospheric pressure. Several other people; Elvius from Sweden (1710), a Christian of Lyons (1743), and the botanist Carolus Linnaeus (1740); also independently invented the same measurement.

Since there are one hundred graduations between these two reference points, the original term for this system was centigrade (100 parts) or centesimal. In 1948 the system's name was officially changed to Celsius (a third name which had also been in use before then) by the 9th General Conference on Weights and Measures (CR 64), both in recognition of Celsius himself and to eliminate confusion caused by conflict with the use of the SI centi- prefix. While the values for freezing and boiling of water remain approximately correct, the original definition is unsuitable as a formal standard: it depends on the definition of standard atmospheric pressure which in turn depends on the definition of temperature. The current official definition of the Celsius sets 0.01 °C to be at the triple point of water and a degree to be 1/273.16 of the difference in temperature between the triple point of water and absolute zero. This definition ensures that one degree Celsius represents the same temperature difference as one kelvin.

Anders Celsius originally proposed that the freezing point should be 100 degrees and that the boiling point should be 0 degrees. This was reversed in 1747, at the instigation of Linnaeus, or perhaps Daniel Ekström , the manufacturer of most of the thermometers used by Celsius.

The Celsius scale is used throughout most of the world for day-to-day purposes, though in broadcast media it was still frequently referred to as centigrade until the late 1980s or early 1990s, particularly by weather forecasters on European networks such as the BBC, ITV, and RTÉ. In the United States and Jamaica, Fahrenheit remains the preferred scale for everyday temperature measurement, although Celsius or kelvin are used for scientific applications. Many people in the United Kingdom still also reckon temperature using the Fahrenheit scale, although Celsius is now used by the government and media. It has become common in the British media to drop the degree ° in speech and in text and refer to a temperature as 21 Celsius, for example, as though Celsius were units and not a scale.