The degree Celsius (°C) is a unit of temperature named after the Swedish astronomer Anders Celsius (1701–1744), who first proposed it 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.

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 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 United States and Jamaica, Fahrenheit remains the preferred scale for everyday temperature measurement, although Celsius or kelvin are used for scientific applications.

A temperature of −40 degrees is the same for Celsius and Fahrenheit. Correspondingly, a method for converting Celsius to Fahrenheit is to add 40, multiply by 1.8, and subtract 40. Similarly, to convert Fahrenheit to Celsius add 40, divide by 1.8, and subtract 40.