Six Sigma is a business methodology for quality improvement that measures how many defects there are in a current process and seeks to systematically eliminate them.

Lean Six Sigma uses the Six Sigma methodology (define, measure, analyze, improve, control) with the specific goal of eliminating waste in a company's, or other organization's, processes or use or materials—that is, making it "leaner." It derives in part from the principles of lean manufacturing.

It's called Six Sigma because the term sigma refers to one standard deviation in a data set. The idea is that six such deviations should occur before the process results in a defect. When a process achieves Six Sigma, it reaches a point where only 3.4 errors per one million process events result in a defect.