A band shape described by the Gaussian function:
In this equation,

is proportional to the width of the band, and

is the frequency of the band maximum.
Source:
PAC, 1996, 68, 2223
(Glossary of terms used in photochemistry (IUPAC Recommendations 1996))
on page 2244
See also:
PAC, 1990, 62, 2167
(Glossary of atmospheric chemistry terms (Recommendations 1990))
on page 2192