Aromatic hydrocarbons or arenes are a type of planar, cyclic (or polycyclic) hydrocarbon whose structure incorporates a type of chemical bonding in which the electrons participating in the bond are delocalized in a particular way.
A rule-of-thumb for recognizing aromaticity is in counting the number of delocalized electrons--if they equal (2n+2), where n is an integer, then the molecule is likely aromatic.
Benzene, C6H6, was recognized as the first aromatic hydrocarbon, with the nature of its bonding first being recognized by Kekule in the 19th Century. He envisioned the delocalised electons rapidly shifting configuration between one of two forms, or resonance structures, in which double bonds rapidly move about the hexagonal ring. However, the total strenth of the aromatic bonds involved in aromaticity is stronger than the total strength of the bonds when formulated as a combination of single and double bonds. Aromatic bonding, therefore, must be recognized as a type of bonding distinct from other types of multiple bonding, such as double or triple bonds. This can be described more quantitatively with molecular orbital theory.
Napthalene, consisting of two coplanar six membered rings sharing an edge, C10H8 is another aromatic hydrocarbon.
Phenol is a common arene.