Quasi-continuous range of discrete energy levels that an electron is either allowed (an occupied band) or forbidden (forbidden band or band gap) to occupy in a molecule or solid.
See also: energy band theory, energy gap
Source:
PAC, 2022, 94, 15. 'Glossary of terms relating to electronic, photonic and magnetic properties of polymers (IUPAC Recommendations 2021)' on page 29 (https://doi.org/10.1515/pac-2020-0501)