The energy in a solid at which the average number
of particles per quantum state is ½; i.e. one
half of the quantum states are occupied.
level in conductors lies in the conduction band, in
insulators it lie in the valence band, and in semiconductors
it falls in the gap between the conduction band and
the valence band.
At absolute zero all the electrons
would occupy energy levels up to Fermi level and no
higher levels would be occupied.