DPM is defects per million. PPM is parts per million, as barb correctly states. While some in industry use these terms interchangeably, it is perhaps incorrect to do so. As barb stated, PPM is used in many other cases as well. One of the most popular is in the measure of contaminants. For example, the impurity level of bottled gases is stated in PPM. If PPM is used to describe defects per million, its use must be clear in the context of the message. With DPM, there is no such reason for clarity. Its use is clear.
Just the ramblings of an Old Wizard Warrior.