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.
Source:
PAC, 1996, 68, 2223
(Glossary of terms used in photochemistry (IUPAC Recommendations 1996))
on page 2233
