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.

The fermi 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.