From AMS Glossary
The most widely used of all the indices of geomagnetic activity.
It is intended as a measure of the worldwide ("p" is for planetary) average level of activity; however, it is very sensitive to certain auroral-zone activity and insensitive to some other types of disturbances. It is based on an index called the K index from each of 12 stations between latitudes 48° and 63°, selected for good longitudinal coverage. The Handbook of Geophysics and the Space Environment (1985) has a discussion of this and other geomagnetic indices.
Jursa, A. S., Ed. 1985. The Geomagnetic Field. The Handbook of Geophysics and the Space Environment. Air Force Geophysics Laboratory, Hanscom AFB, MA, . chap. 4.