A saturated organic compound has general formula #C_nH_(2n+2)#. Halides count for #H#, and if the formula contains nitrogen, we subtract #NH# before the determination. Every #2H"'s"# less than the saturated formula corresponds to #"a degree of unstaturation"#. And each degree of unsaturation corresponds to either a double bond (i.e. #C=O#, #C=C#, #C=N#) OR a ring junction.
For benzene, the molecular formula is #C_6H_6#. A saturated alkane with 6 carbons would have a formula of #C_6H_14#. Thus benzene has #4# degrees of unsaturation. The representation of benzene as #"1,3,5-cyclohexatriene"# makes these degrees of unsaturation very easy to see.
So aromatic compounds are by definition unsaturated.
