1. The minimum energy needed to remove an electron from the Fermi level (average energy of electrons) of a metal to infinity; usually expressed in electrovolts.
2. The energy difference between the Fermi level and vacuum zero.

The minimum amount of energy it takes to remove an electron from a substance into the vacuum.

The term Fermi refers to a unit of length, equal to 10^-15 meters; used particularly in measuring nuclear distances.