It's a statistical term. (You'll find 'sigma' in all manner of standard deviation calculations - or, for the less mathematical, the capital sigma is that sign in Excel that adds everything up.) It refers to instances of defects in a production or services environment - usually relating to a process - such as that there are no more than 3.4 defects per million instances. Five or four sigma would be higher levels of defects but six is the standard.
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