What does the term 'Six Sigma' mean?

The term "sigma", taken from the Greek alphabet, is used in statistics as a measure of deviation. If a company achieves a "six sigma" measurement in production, for example, that means that only 3.4 of 1 million products are defective. This represents a 99.9997 percent error-free performance. Not all Six Sigma projects reach that lofty goal but they aspire to it and set continuous improvement as a standard.