Ppk

A measurement of the long-term process performance (a.k.a. voice of the process or VOP) in relation to the spread (or total tolerance) between the customer's lower/upper specification  limits (LSL/USL) (a.k.a. the voice of the customer or VOC). It's used for measuring process capability where if it's less than 1, then the process is not capable within the LSL/USL tolerance. The higher it is above 1, the more capable the process is of achieving results within tolerance. It is calculated as the minimum(Z_usl/3, Z_lsl/3). Since it represents long-term data, it will always be lower than Cpk (which represents short-term data); if it's significantly lower than Cpk, then it's likely driven by the long term variation (mean shift) between sub-groups.
Related StatStuff Video: 
   -Section 6-Analyze Phase, Lesson #5 - Process Capability: Step 4 (Normal Distributions)