Six Sigma
It is a philosophy for managing process improvements. It’s also a methodology of structured problem solving, applying associated toolsets to reduce process variations and thereby eliminating defects. As a standard metric, Six Sigma quality is 3.4 defects per million opportunities.

Special Causes Variation
Variation that is due to an identifiable, out-of-the-ordinary event; it is not a usual part of the process.

Statistical Process Control (SPC)
Statistical Process Control is a system that utilizes control charts to keep a process operating within its appropriate variation limits. SPC was created by Walter Shewhart of Bell Labs in the early 1920s.