What is Celsius ?
Celsius is adj,(noun) a scale of temperature where the freezing point of water is 0 degrees and the boiling point is 100 degrees The temperature outside is only 6 degreesC (say ‘six degrees Celsius’). What is 75 degrees Fahrenheit in Celsius? (NOTE: used in many countries, but not in the USA, where the Fahrenheit system is still preferred. It is usually written as a C after the degree sign: 32 degrees C (say: ‘thirty-two degrees Celsius’). It was formerly called centigrade.)
source: Easier English, Student Dictionary Upper Intermediate Level