Carat is the measure of weight of a diamond. 
The last of the 4C’s is carat weight. Diamonds are measured in "carats", each carat being 100 points. For example, a half carat is 50 points and is expressed as 0.50 carats.  All things being equal, the greater the carat weight, the rarer the diamond, the more expensive it is. As diamonds increase in size, their cost increases geometrically rather than arithmetically. Thus, a one-carat diamond may cost more than twice as much as a one-half carat diamond of equal quality. 

Two diamonds of equal weight can vary widely in price because of differences in the quality of their cut, clarity and color. A smaller diamond may actually be more valuable than a larger one.