TIA's Glossary of Telecommunication Terms
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spectral width
The wavelength interval over which the magnitude of all spectral components is equal to or greater than a specified fraction of the magnitude of the component having the maximum value. (188) Note 1: In optical communications applications, the usual method of specifying spectral width is the full width at half maximum. This method may be difficult to apply when the spectrum has a complex shape. Another method of specifying spectral width is a special case of root-mean-square deviation where the independent variable is wavelength, 
, and f () is a suitable radiometric quantity. Note 2: The relative spectral width , 
/, is frequently used where is obtained according to note 1, and is the center wavelength.