Six Sigma is a disciplined, data-driven approach and methodology for eliminating defects in any process -- from manufacturing to transactional and from product to service.

The goal in Six Sigma projects is to drive process quality towards having fewer rejects/error than six standard deviations from the mean.

This is accomplished through the use of two Six Sigma sub-methodologies: DMAIC and DMADV. The Six Sigma DMAIC process (define, measure, analyze, improve, control) is an improvement system for existing processes falling below specification and looking for incremental improvement. The Six Sigma DMADV process (define, measure, analyze, design, verify) is an improvement system used to develop new processes or products at Six Sigma quality levels. It can also be employed if a current process requires more than just incremental improvement.

According to the General Electric website, "Six Sigma is a highly disciplined process that helps us focus on developing and delivering near-perfect products and services. Why 'Sigma'? The word is a statistical term that measures how far a given process deviates from perfection. The central idea behind Six Sigma is that if you can measure how many 'defects' you have in a process, you can systematically figure out how to eliminate them and get as close to 'zero defects' as possible."

from http://www.isixsigma.com/sixsigma/six_sigma.asp
http://www.ge.com/sixsigma/