The degree Celsius (symbol: °C) is a unit of temperature approved for use with the SI. The degree Celsius is equal to exactly one kelvin, which is defined as 1/273.16 of the thermodynamic temperature of the triple point of water. The Celsius or centigrade scale is related to the kelvin (absolute) scale by setting the temperature zero degrees Celsius (0 °C) to be exactly 273.15 K, thus absolute zero is -273.15 °C.
The original centigrade scale was developed by Carolus Linnaeus in 1744 and set the temperature of the melting point of water (at atmospheric pressure) to 0 °C, and the boiling point of water (at atmospheric pressure) to 100 °C. The Celsius scale is named for Anders Celsius (1701 – 1744), a Swedish astronomer, who developed a temperature scale similar to Linnaeus' scale in 1742, but with zero at the boiling point of water, and 100 degrees at the melting point.
Due to the redefinition of the degree Celsius and the definition of "standard atmospheric pressure", the melting point of ice at standard atmospheric pressure is no longer 0 °C, but is 0.002519 °C, and the boiling point of water at standard atmospheric pressure is not exactly 100 °C, but is estimated as 99.9839 °C