In a semiconductor, the range of electron energy, higher than that of the valence band, sufficient to make the electrons free to move from atom to atom under the influence of an applied electric field and thus constitute an electric current. 2. In the atomic structure of a material, a partially filled or empty energy level in which electrons are fr [..]

the unfilled energy levels into which electrons can be excited to become conductive electrons; a band that when partially occupied by mobile electrons, permits their net movement in a particular direc [..]

A partially filled energy band in which electrons can move freely.