DPMO

It stands for Defects per Million  Opportunities and is a count of the number of defects expected to occur for every one million opportunities run through the  process. It's essentially like a percent defective or p(d) that's carried out to the 4th decimal place. The equation to calculate it is (Total Defects / (Total Units x Opportunities per Unit)) x 1,000,000.
Related StatStuff Video: 
   -Section 6-Analyze Phase, Lesson #5 - Process Capability: Step 4 (Normal Distributions)