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 an 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. 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; nonbenzenoid aromatic; pseudoaromatic.
http://www.acdlabs.com/iupac/nomenclature/79/r79_61.htm Information about IUPAC nomenclature