Six sigma history

Since the 1920's the word 'sigma' has been used by mathematicians and engineers as a symbol 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 an informal name for an in-house initiative for reducing defects in production processes, because it represented a suitably high level of quality. (Note here it's Sigma with a big 'S' because in this context Six Sigma is a 'branded' name for Motorola's initiative.)
(Certain engineers - there are varying opinions as to whether the very first was Bill Smith or Mikal Harry - felt that measuring defects in terms of thousands was an insufficiently rigorous standard. Hence they increased the measurement scale to parts per million, described as 'defects per million', which prompted 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 the late-1980's following the success of the above initiative, 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, ie., beyond purely 'defect reduction', in Motorola Inc.
In 1991 Motorola certified its first 'Black Belt' Six Sigma experts, which indicates the beginnings of the formalization of the accredited training of Six Sigma methods.
In 1991 also, Allied Signal, (a large avionics company which merged with Honeywell in 1999), adopted the Six Sigma methods, and claimed significant improvements 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 approached Motorola's CEO Bob Galvin to learn how it could be used in Allied Signal.