a measure of the energy of the least tightly held electrons within a solid, named for Enrico Fermi (Fermi, Enrico), the physicist who first proposed it. It is important in determining the electrical and thermal properties of solids. The value of the Fermi level at absolute zero (−273.15 °C) is called the Fermi energy and is a constant for each solid. The Fermi level changes as the solid is warmed and as electrons are added to or withdrawn from the solid. Each of the many distinct energies with which an electron can be held within a solid is called an energy level. According to the laws of quantum mechanics, each energy level can accommodate only a limited number of electrons. The Fermi level is any energy level having the probability that it is exactly half filled with electrons. Levels of lower energy than the Fermi level tend to be entirely filled with electrons, whereas energy levels higher than the Fermi tend to be empty.

When materials with different individual Fermi levels are placed in contact, some electrons flow from the material with the higher Fermi level into the other material. This transfer of electrons raises the lower Fermi level and lowers the higher Fermi level. When the transfer is complete, the Fermi levels of the two materials are equal. This behaviour is important in electronic devices that juxtapose different materials.