Erdős number
Paul Erdős (1913–1996) was a Hungarian mathematician. He considered mathematics to be a social activity and often collaborated on his papers, having 511 joint authors, many of whom also have their own collaborators. The Erdős number measures the "collaborative distance" between an author and Erdős. Thus, his direct co-authors have Erdős number one, theirs have number two, and so forth. Erdős himself has Erdős number zero.