Celsius v. Centigrade

I hear people use the both terms, seemingly interchangeably, on a semi-regular basis. It seems funny to me that either should be tossed around with much frequency here in the U.S., where imperial and avoirdupois systems of measurement reign supreme.

The Short:

The Celsius and centigrade temperature scales are the same temperature scales where zero degrees occurs at the freezing point of water and one hundred degrees is at the boiling point of water. However, the Celsius scale uses a zero that can be precisely defined.

The Long:

Anders Celsius, a professor of astronomy at the University of Uppsala, Sweden, devised a temperature scale in 1741. His original scale had 0 degrees at the point where water boiled and 100 degrees at the point where water froze. Because there were 100 degrees between the defining points of the scale, it was a type of centigrade scale. Upon Celsius' death, the endpoints of the scale were switched (0°C was the freezing points of water; 100°C was the boiling point of water) and the scale became known as the centigrade scale. 

How Centigrade Became Celsius

The confusing part here is that the centigrade scale was invented by Celsius, more or less, so it had been called Celsius' scale or the centigrade scale. However there were a couple of problems with the scale. First, the grade was a unit of plane angle, so a centigrade could be one-hundredth of that unit. More importantly, the temperature scale was based on an experimentally determined value which could not be measured with the precision deemed sufficient for such an important unit. In the 1950s, the General Conference of Weights and Measures set out ot standardize several units and decided to define Celsius temperature as kelvin minus 273.15. The triple point of water was defined to be 273.16 kelvin and 0.01°C. The triple point of water is the temperature and pressure at which water exists simultaneously as a solid, liquid and gas. The triple point can be measured accurately and precisely, so it was a superior reference to the freezing point of water. Since the scale had been redefined, it was given a new official name, the Celsius temperature scale.

Information from Chemistry.About.com