'physicist Enrico Fermi asked his students at the University of Chicago, “How many piano tuners are there in Chicago?” By asking such questions, Fermi wanted his students to make estimates that involved rough approximations so that their goal would be not precision but the order of magnitude of their result. Thus, many people today call these kinds of questions “Fermi questions”.'

http://www.vendian.org/envelope/dir0/fermi_questions.html