Celsius
Celsius is a measurement of temperature. It was originally defined to be where Celsius is where water freezes at 1 atm and Celsius is where water boils at 1 atm.
However different thermometers register slightly different temperatures at these points, so now, two points: the absolute zero and the triple point of water are used to fix the Celsius scale. To avoid incompatibility, absolute zero is defined to be and the triple point is .
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