A six-sigma process is one in which 99.99966% of the products manufactured are free of defects (compared to a one-sigma process in which only 31% are free of defects). This amounts to only 3.4 defective products that are manufactured. In Six Sigma, a defect is defined as any process output that does not meet customer specifications, or that could lead to creating an output that does not meet customer specifications. So, the goal of Six Sigma is to increase profits by eliminating "defects" that undermine customer loyalty.
DFSS - Design for Six Sigma
DMADV - Define, Measure, Analyze, Design, Verify
DMAIC - Define, Measure, Analyze, Improve, Control
FMEA - Failure Modes and Effects Analysis