This text was copied from this Peakbagger glossary page. I made a separate page for it so the text was easier to find.
The P-Index for a climber is the number N, such that the climber has climbed N peaks with N meters of prominence or more. For example, a climber with a P-Index of 300 has climbed 300 peaks with over 300 meters of prominence. It is a rough measure of the number and quality of prominent peaks a climber has summitted.
Since there are approximately 1500 peaks on Earth with 1500 meters of prominence, a P-Index of 1500 is the maximum value attainable. This symmetry is also why the index is calculated using meters, instead of feet. Additionally, note that this site uses a peak's clean prominence value for calculating a climber's P-Index. Any climber with a P-Index over about 500 can be considered an exceptionally prolific prominence-oriented peakbagger.
The P-Index was proposed by Lee Newton and is based on the H-Index concept that is used to track the number and quality of a scientist's research papers.
Note that on this site the P-index was calculated using average prominence in the past, but has been updated to use clean prominence. Clean is the default prominence used throughout Peakbagger.com and for the sake of consistency the P-Index now does the same.