Resonance: It is a process of comparing and contrasting two or more dissimilar Lewis dot structures that can perhaps represent a molecule. They show resonance property from X-ray analysis. We see that the length of C-C bond of aromatic compound (benzene) is in between the length of normal single & double bond. It is 0.139nm while double-bond length is 0.134 nm & single bond length is 0.154 nm.
The resonance effect is a chemical phenomenon observed in compounds characteristic of double bonds of organic compounds.