A vacant or only partially occupied set of many closely spaced electronic levels resulting
from an array of a large number of atoms forming a system in which the electrons can
move freely or nearly so. This term is usually used to describe the properties of
metals and semiconductors.
PAC, 1996, 68, 2223
(Glossary of terms used in photochemistry (IUPAC Recommendations 1996))
on page 2233