The number of chances for a per unit of a product/process to fail to meet a Critical To Quality (CTQ) requirement. The total number of Defect opportunities reflects the complexity of the product/process and thus facilitates a more direct comparison between processes with varying levels of complexity than the Defects per unit (DPU) metric. For instance, a car assembly process has far more opportunities for error than a widget manufacturing process, so Defects per unit is a poor basis for comparing the capabilities of the two processes.
Consider a simple process: Filling out an order form.
Let's say there are 20 fields to be filled out, each of which could result in a defect (incorrect or left blank). The number of defect opportunities per unit (form) here would be 20 (one for each field).