Six Sigma is a statistical concept that measures a process in terms of defects. Achieving "Six Sigma" means your processes are delivering only 3.4 defects per million opportunities (DPMO) - in other words, they are working nearly perfectly. Sigma (the Greek letter σ) is a term in statistics that measures standard deviation. In its business use, it indicates defects in the outputs of a process, and helps us to understand how far the process deviates from perfection.