The Collaborative International Dictionary of English v.0.48:

Benzene \Ben"zene\, n. [From Benzoin.] (Chem.)
   A volatile, very inflammable liquid, C6H6, contained in the
   naphtha produced by the destructive distillation of coal,
   from which it is separated by fractional distillation. The
   name is sometimes applied also to the impure commercial
   product or benzole, and also, but rarely, to a similar mixed
   product of petroleum.
   [1913 Webster]

   Benzene nucleus, Benzene ring (Chem.), a closed chain or
      ring, consisting of six carbon atoms, each with one
      hydrogen atom attached, regarded as the type from which
      the aromatic compounds are derived. This ring formula is
      provisionally accepted as representing the probable
      constitution of the benzene molecule, C6H6, and as the
      type on which its derivatives are formed.
      [1913 Webster]

WordNet (r) 3.0 (2006):

benzene nucleus
    n 1: a closed chain of 6 carbon atoms with hydrogen atoms
         attached [syn: benzene formula, benzene ring, benzene
         nucleus, Kekule formula]