Celsius

The Celsius (also Centigrade) scale is a was measuring temperature. Originally, $0^\circ\mathrm{C}$ was defined as the temperature at which water freezes at 1 atm, and $100^\circ\mathrm{C}$ as the temperature at which water boils at 1 atm.

However, since different thermometers register slightly different temperatures at these points, the absolute zero and the triple point of water are used to fix the Celsius scale. They are defined, respectively, to be $-273.15^\circ\mathrm{C}$ and $0.01^\circ\mathrm{C}$ .