Erdős number
Considering Paul Erdős to have an index=0, people who co-authored with him have an index=1, co authors of those co-authors have index=2, and so on. Thus, to calculate one’s Erdős number, add 1 to the Erdős number of any co-author with the lowest Erdős, number. The Erdős-Number project at Oakland University maintains a website tracking the Erdős numbers of scientists world-wide.