Six Sigma is a quality-control methodology developed in 1986 by Motorola, Inc. The method uses a data-driven review to limit mistakes or defects in a corporate or business process. Six Sigma emphasizes cycle-time improvement while at the same time reducing manufacturing defects to a level of no more than 3.4 occurrences per million units or events. In other words, the system is a method to work faster with fewer mistakes.
Six Sigma points to the fact that, mathematically, it would take a six-standard-deviation event from the mean for an error to happen. Because only 3.4 out of a million randomly (and normally) distributed, events along a bell curve would fall outside of six-standard-deviations (where sigma stands in for "standard deviation").