Sigma is a letter in the Greek Alphabet. As a letter (∑), Sigma is used as a symbol for sums and series in which each term is computed from the previous one by adding (or subtracting).
Six Sigma is a Business Management Strategy which was developed by Biill Smith who is also referred as father of Six Sigma. He was working in Motorola,USA, when he developed this methodology/ strategy (in 1981). Six Sigma is a methodology for continuous improvement using which process centering or variation between parts is controlled to bring process efficiency and reduce defects. We can also refer this as problem solving technique using statistical approach. It improves the quality of any process by identifying and removing the causes of defects and reducing variability. When we say a process is six sigma, we mean process accuracy is 99.9997% and has an opportunity of only 3.4 defects per million.
The true meaning of Six Sigma is the center of the process is away from the nearest specification limit (USL or LSL) by six standard deviations/Sigma of the process. Six Sigma not just helps you improve your process performance by reducing cost of poor quality and cost of quality but also improves your business sales and thus improving revenue and profits by providing competitive edge to your company in enhancing customer satisfaction. Higher sigma value for your process would indicate lower variation and thus producing fewer defects. Once your confidence in the process goes up, you can reduce cost of quality by monitoring only statically important sample instead of entire lot. Monitoring of sample is still required to catch increase in variation and ensure process stays on track.
Six Sigma uses 2 approaches/ structured method depending upon if you are making improvement on existing process or working on new product or designing process.
As per the Six Sigma Academy, Black Belts save companies approximately $230,000 per project and can complete four to six projects per year. Companies like GE which is considered to be successful in implementing Six Sigma has been estimated to saved over $10 Billion in first 5 years of implementation.