[r]efers loosely to striving for near perfection in the performance of a process or production of a product. The name derives from the Greek letter sigma, often used to refer to the standard deviation of a normal distribution. By definition, 95% of a normally distributed population falls within 2 standard deviations of the average (or "2 sigma"). This leaves 5% of observations as "abnormal" or "unacceptable." Six Sigma targets a defect rate of 3.4 per million opportunities — 6 standard deviations from the population average.
The term "six sigma" relates to the statistical function used to measure the amount of variance in a process. Six Sigma as an industry program is an effort to achieve high quality, low cost, and minimum cycle time, resulting in a highly satisfied customer. Six Sigma is a way to measure the chance that any unit of product can be manufactured with zero defects — no more than 3.4 defects per one million items. To achieve a Six Sigma design, the characteristics of the design that most affect performance and reliability need to be identified. Then, by increasing the allowable range of variation of those characteristics, many more units will be usable. This is a six-step process depending on customer requirements and process capability.