The term "Six Sigma" is a reference to a particular goal of reducing defects to near zero. Sigma is the Greek letter statisticians use to represent the "standard deviation of a population." The sigma, or standard deviation, tells you how much variability there is within a group of items. In statistical terms, therefore, the purposes of Six Sigma is to reduce variation to reduce variation to achieve very small standard variations so that almost all of your products or services meet or exceed customer expectations

p. 4 - The six sigma way, 2000