One of the
\(3N - 6\) (
\(3N - 5\) for a linear molecule)
modes of vibration that would add to give the total vibrational motion in a molecule if the interatomic potential were strictly quadratic in the displacements from equilibrium.
Note: Normal modes of vibration is an idealized concept that has proved extremely useful in analysing spectra in
vibrational spectroscopy.
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 757 (https://doi.org/10.1515/pac-2019-0203)