Daniel Lemire sparked my interest in finding this out. The link above is to a page under the Erdös Number Project. Paul Erdös was a prolific mathematician, and an Erdös number is the number of "degrees of separation" one is away from him. As that site says, the vast majority of mathematicians have an Erdös number less than eight. So, one of his co-authors has an Erdös number of 1, a co-author of a co-author has an Erdös number of 2, etc. See this Wikipedia entry for more. You can use MathSciNet to help you computer your Erdös number, but as it focuses on mathematics publications, it only gives an upper bound (for instance, it gives me an Erdös number of 6, but one of the folks along the pathway, listed as a co-author of my PhD advisor, is also one of my co-authors). I can actually trace multiple paths of length 5 to Paul Erdös. And I don't consider myself a mathematician; I just hang out with the wrong people.