The idea that an
electronic transition occurs much faster than
nuclear motion. For example, if you hit a
molecule with a
photon, driving it into the
excited state, the electrons will rearrange their distribution in the orbitals without the
nuclei having a chance to rearrange themselves. The resulting system is often not in equilibrium, and depending on the lifetime of the excited state, the nuclei may then rearrange (slightly) to accomodate the new electron distribution.