Six Sigma is a set of tools and strategies for process improvement originally developed by Motorola in 1985.[1][2] Six Sigma became well known after Jack Welch made it a central focus of his business strategy at General Electric in 1995,[3] and today it is used in different sectors of industry.[4]
Six Sigma seeks to improve the quality of process outputs by identifying and removing the causes of defects (errors) and minimizing variability in manufacturing and business processes.[5] It uses a set of quality management methods, including statistical methods, and creates a special infrastructure of people within the organization (“Champions”, “Black Belts”, “Green Belts”, “Orange Belts”, etc.) who are experts in these very complex methods.[5]
Each Six Sigma project carried out within an organization follows a defined sequence of steps and has quantified value targets, for example; process cycle time reduction, customer satisfaction, reduction in pollution, cost reduction and/or profit increase.[5]
The term Six Sigma originated from terminology associated with manufacturing, specifically terms associated with statistical modeling of manufacturing processes. The maturity of a manufacturing process can be described by a sigma rating indicating its yield or the percentage of defect-free products it creates. A six sigma process is one in which 99.99966% of the products manufactured are statistically expected to be free of defects (3.4 defects per million), although, as discussed below, this defect level corresponds to only a 4.5 sigma level. Motorola set a goal of “six sigma” for all of its manufacturing operations, and this goal became a byword for the management and engineering practices used to achieve it.

Reference: https://en.wikipedia.org/wiki/Six_Sigma