Contrary to popular belief "Six Degrees of Separation" wasn't a game Kevin Bacon's fan club invented, but an application to the idea that everyone anywhere in the world is approximately six steps away from any other human being.
The idea was initiated in "Chains" (1929); a short story by Hungarian writer Frigyes Karinthy. The story would later inspire scientists to put the idea into research and the experiments showed that people in USA were chained by three friendship links.
Then came along playwright John Guare whose homonym play added the "six" in the term and familiarized people with the concept. Since then applications and games grew by dozens, such as "Bacon number" that finds the connection between Bacon and any another Hollywood actor.
One of them is "Erdős number" in honor to the mathematician Paul Erdős who has published the most papers in his field. Itinerant Erdős co-writed at least 1400 papers with colleagues (those have Erdős number: 1) that provided him food and shelter. Erdős number "calculates" the authoring distance between Erdős and the authors that have collaborated with Erdős direct co-writers. For example, philosopher Noam Chomsky has Erdős number: 4, which means that he has collaborated with someone with an Erdős number: 3, who was collaborated with someone with an Erdős number: 2, who was collaborated with a person that has directly worked with Erdős himself.
Trivia: Few people in the world have an Erdős-Bacon number.