Since the 1920's the word 'Sigma' has been used by statisticians and engineers as a mark for a unit of measurement in product quality variation. (Note it's Sigma with a small 's' because in this context Sigma is a generic unit of measurement.)
In the mid-1980's engineers in Motorola Inc in the USA used 'Six Sigma' an informal name for an in-house creativity for decreasing defects in production processes, because it represented a suitably high level of quality.
Certain engineers - there are changeable opinions as to whether the very first was Bill Smith or Mikal Harry - felt that quantifying defects regarding thousands were an insufficiently rigorous standard. Hence, they increased the measurement scale to parts per million, described as 'defects per million', which encouraged the use the 'six sigma' terminology and adoption of the capitalised 'Six Sigma' branded name, given that six sigma was deemed to equate to 3.4 parts - or defects - per million.
In-1980's the success of the above creativity, Motorola extended the Six Sigma methods to its critical business processes, and significantly Six Sigma became a formalized in-house 'branded' name for a performance improvement methodology, i.e., beyond purely 'defect reduction,' in Motorola Inc.
In 1991 Motorola certified its first 'Black Belt' Six Sigma experts, which specifies the beginnings of the formalization of the accredited training of Six Sigma methods.
In 1991 also, Allied Signal adopted the Six Sigma methods and claimed significant progress and cost savings within six months. It seems that Allied Signal's new CEO Lawrence Bossidy learned of Motorola's work with Six Sigma and so loomed Motorola's CEO Bob Galvin to learn how it could be used in Allied Signal.
In 1995, General Electric's CEO Jack Welch decided to implement Six Sigma in GE, and by 1998 GE claimed that Six Sigma had generated over three-quarters of a billion dollars of cost savings.
By the mid-1990's Six Sigma had developed into a convenient 'branded' corporate management initiative and methodology, notably in General Electric and other large manufacturing corporations, but also in organizations outside the manufacturing sector.
By the year 2000, Six Sigma was effectively established as an industry in its right, involving the training, consultancy and six sigma implementation of Six Sigma systems in all sorts of organizations around the world.
That is to say, in a little over many years; Six Sigma speedily became not only a hugely popular methodology used by many organizations for quality and process improvement, Six Sigma also became the subject of many and various training and consultancy products and services around which developed very many Six Sigma support organizations.