The term spectral band can be used in a general way
to indicate a contiguous set (interval) of spectral
wavelengths. It is often shortened to the term band.
A common use is to describe the wavelength-range of filters
or receivers.
Spectral band is also used assuming a more specific meaning:
such an interval of wavelengths that are affected by
a molecule, often a large, complex molecule,
that produces many spectral lines, often forming a
closely-packed pattern, even to the point of overlapping.
If they overlap or if the spectroscope's
resolution is low, it is seen as a
range of absorption (absorption band) or
emission (emission band).
Polycyclic aromatic hydrocarbons (PAHs) produce these, e.g., PAH emissions.
Water lines are another example.