A metric unit of temperature. The Celsius temperature scale is named for the Swedish astronomer and physicist Anders Celsius (1701-1744), who used a similar scale. The freezing point of water (at one atmosphere of pressure) was originally defined to be 0 °C, while the boiling point is 100 °C. Thus the Celsius degree is 1/100 of the difference between these two temperatures. (The scale actually used by Celsius was inverted, so that 0 °C was the boiling point of water and 100 °C the freezing point.) In the SI system, the Celsius scale is defined so that the temperature of the triple point of water (the temperature at which water can exist simultaneously in the gaseous, liquid, and solid states) is exactly 0.01 °C, and the size of the degree is 1/273.16 of the difference between this temperature and absolute zero (the temperature at which all molecular motion ceases). For practical purposes this is equivalent to the original definition. See also: Degree Fahrenheit, Kelvin