- Kevin Bacon has a Bacon Number of zero
- An actor/actress whom has appeared in a movie with Kevin Bacon has a Bacon number of 1
- The Bacon number of any individual actor/actress is then is defined as the minimum of the Bacon numbers of all the co-actor/actresses he/she has appeared in a movie in, plus one
The concept got an honourable mention at this years sci/tech Oscars ceremony; it's probably too much to hope that it'll get a mention at the main awards this weekend.
Incidentally, the concept of the Bacon Number is derived from the Erdős number, which is the minimum distance by co-authors via publications to the late mathematician Paul Erdős. Blowing my own trumpet, at the moment, I have an Erdős number of 6 (see here - requires mathscinet access). The link was provided via a paper I co-authored with Mikhail Propenko, who has an Erdos number of 5. There is such a thing as an Erdős-Bacon number; informally, the lowest number is, in fact, held by an inanimate Baseball (in rod we trust!).
Degrees of separation feature strongly in research into social networking, often going hand-in-hand with work in graph theory. The nifty thing about it is its broad area of application; an example of which is the Wikipedia degrees of separation calculator which has been sucking up a lot of my nonexistent spare time :P