From AMS Glossary
A connected series of closely spaced or overlapping absorption lines.
Absorption bands are common features in the absorption spectra of polyatomic gases. Absorption bands arise when absorbed photons are converted to simultaneous changes in more than one of the electronic, vibrational, or rotational molecular energy states. Changes to vibrational energy are usually accompanied by changes in rotational energy and give rise to vibrational-rotational absorption bands. Similarly, changes to molecular electron levels yield electronic-vibrational-rotational absorption bands.