An organic compound that contains a benzene ring in its molecules or that has chemical properties similar to benzene. Aromatic compounds are unsaturated compounds, yet they do not easily partake in addition reactions. Instead they undergo electrophilic substitution.

Benzene, the archetypal aromatic compound, has a hexagonal ring of carbon atoms and the classical formula (the Kekulé‎ structure) would have alternating double and single bonds. In fact all the bonds in benzene are the same length intermediate between double and single C–C bonds. The properties arise because the electrons in the π‎-orbitals are delocalized over the ring, giving an extra stabilization energy of 150 kJ mol^-1 over the energy of a Kekulé‎ structure. The condition for such delocalization is that a compound should have a planar ring with (4n + 2) pi electrons – this is known as the Hückel rule. The Hückel rule was derived in 1931 by Erich Hückel using molecular-orbital theory. It was derived in 1963 using valence-bond theory. The Hückel rule breaks down for n greater than 6 due to the system becoming like a chain in which there is a Peierls instability. Aromatic behaviour is also found in heterocyclic compounds such as pyridine. Aromatic character can be detected by the presence of a ring current using NMR. See also annulenes; Heillbronner’s rule; nonbenzenoid aromatic; pseudoaromatic.

https://www.acdlabs.com/iupac/nomenclature/79/r79_61.htm Information about IUPAC nomenclature