One virtue of Six Sigma is that it translates the messiness of variation of variation into a clear black or white measure of success: either a product or service meets customer requirements or it doesn't. Anything that does not meet customer requirements is called a defect. [One] approach to determining a sigma level is to calculate how many defects occur per opportunities for things to go wrong. The outcome of this calculation is called Defects per Million.

p. 6 - The six sigma way, 2000