The energy difference between the bottom of the conduction band and the top of the valence band in a semiconductor or an insulator.
See: conduction band, Fermi level
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2229 (https://doi.org/10.1351/pac199668122223)