Annulenes are monocyclic hydrocarbons that contain the maximum number of non-cumulated or conjugated double bonds (mancude). They have the general formula CH (when n is an even number) or CH (when n is an odd number). The IUPAC accepts the use of annulene nomenclature in naming carbocyclic ring systems with 7 or more carbon atoms, using the name [n]annulene for the mancude hydrocarbon with n carbon atoms in its ring, though in certain contexts (e.g., discussions of aromaticity for different ring sizes), smaller rings (n = 3 to 6) can also be informally referred to as annulenes. Using this form of nomenclature 1,3,5,7-cyclooctatetraene is [8]annulene and benzene is [6]annulene (and occasionally referred to as just annulene).
The discovery that [18]annulene possesses a number of key properties associated with other aromatic molecules was an important development in the understanding of aromaticity as a chemical concept. 
In the related annulynes, one double bond is replaced by a triple bond.