From charlesreid1

https://en.wikipedia.org/wiki/Fermi_problem

A Fermi problem is a type of problem that is intended to hone dimensional analysis skills and back of the envelope estimation.

Enrico Fermi was good with approximate calculations, even when he had little or no data.

Long list here: http://www.physics.umd.edu/perg/fermi/fermi.htm

Interesting economics application: http://www.people.hbs.edu/mweinzierl/paper/dynamicscoring.pdf