The Drake equation is a mathematical equation used to estimate the number of detectable extraterrestrial civilizations in the Milky Way galaxy. It is used in the field of the Search for ExtraTerrestrial Intelligence (SETI). The equation was devised in 1961 by Frank Drake, Emeritus Professor of Astronomy and Astrophysics at the University of California, Santa Cruz.

http://en.wikipedia.org/wiki/Drake_equation