@Louise Oh! I believe that "mesomèrie" is likely equivalent to the English term "resonance", which has a rough synonym in the now obsolete term "mesomerism". So it would refer to the way in which the electron distribution of a molecule can be thought of as the combination of a number of different "resonant structures", e.g. in between these two structures for the nitrite ion, NO₂⁻ https://witches.town/media/1bHO1MNoLXOPlmPO0Hw