In 1742, the Swiss chemist Anders Celsius sought to invent a scale of temperature that would be simple, reproducible, and understandable. He used as his standards to temperatures that (he thought) should be easy to reproduce: the melting point and boiling point of water at a pressure of one standard atmosphere.
The zero of the Celsius scale is the melting or "freezing" point of water at one standard atmosphere.
The boiling point of water at one standard atmosphere is set at 100 °C.
Hence the name "centigrade," for literally one hundred degrees.
Originally the zero was the boiling point, and 100 was the freezing point, until Carolus Linnaeus reversed the numbers to produce the Celsius scale we know today. The definitions were fixed in 1954.
Today, however, the Celsius scale depends on the definition of the kelvin, which is the same interval as a Celsius degree but which is a true proportional temperature. A Kelvin is now defined as 1/273.16th of the temperature at the triple point of water. The zero of the Kelvin scale is absolute zero, the temperature at which all molecular motion stops, the lowest temperature that any substance can ever achieve.