Definitions for "SIX SIGMA"
Sigma is a letter in the Greek alphabet. The term "sigma" is used to designate the distribution or spread about the mean (average) of any process or procedure. For a business or manufacturing process, the sigma value is a metric that indicates how well that process is performing. The higher the sigma value, the better. Sigma measures the capability of the process to perform defect-free-work. A defect is anything that results in customer dissatisfaction. Sigma is a statistical unit of measure which reflects process capability. The sigma scale of measure is perfectly correlated to such characteristics as defects-per-unit, parts-per million defective, and the probability of a failure/error. Meaning no more than 3.4 parts per Million.
A methodology and set of tools used to improve quality to than 3.4 defects per million or better.
Most often this term refers to both the measure of process variation relative to customer specifications and to a method for reducing that variation. You may see Six Sigma written as 6 Sigma or 6.