The approximation of the full nuclear potential of a molecular system in its equilibrium geometry and in the vicinity of the respective minimum on the potential energy surface by
the function
where
are mass-weighted cartesian displacements of nuclei relative to their equilibrium
positions. The approximation allows one to describe vibrational motion in terms of
independent vibrational modes (normal modes) each of which is governed by a simple
one-dimensional harmonic potential.

Source:

PAC, 1999, *71*, 1919*
(Glossary of terms used in theoretical organic chemistry)
* on page 1941