Statistical Meaning of Six Sigma

the term sigma is a Greek alphabet letter (σ) used to describe variability. In Six Sigma, the common measurement index is DPMO (Defects Per Million Operations) and can include anything from a component, piece of material, or line of code, to an administrative form, time frame or distance.
A sigma quality level offers an indicator of how often defects are likely to occur, where a higher sigma quality level indicates a process that is less likely to create defects.
Consequently, as sigma level of quality increases, product reliability improves, the need for testing and inspection diminishes, work in progress declines, cycle time goes down, costs go down, and customer satisfaction goes up.