Resonance is a method for mathematically describing the distribution of electrons in a molecular species with delocalised electrons using a combination of electronic distributions in hypothetical structures with localised bonds.
1. The term "resonance" is now more often used than the term "mesomerism" which refers to the method originally used for treating the delocalised systems
2. The difference between the energy of a real species and the most stable of the hypothetical structures with localised bonds is called the "resonance energy".