The SI unit for temperature is Kelvin, both Celcius and Fahrenheit are based on Kelvin as is any arbitrary temperature unit.
Though, I can probably agree I'm my wording wasn't the best. Fahrenheit is defined using the Kelvin scale. It is based on the temperature of a solution of brine made from equal parts of ice, water and salt (ammonium chloride) and a best estimate of the average human body temperature (96 °F, about 2.6 °F less than the modern value due to a later redefinition of the scale).
As you can see, the scale isn't very good as these units aren't absolute.
As is Celcius. Water freezes at different temperatures depending on pressure, purity, altitude and other factors.