The term spectral band can be used in a general way to indicate a contiguous set (interval) of spectral wavelengths.
It also has 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 produce these, e.g., PAH emissions. Water lines are another example.