Aromaticity is the phenomenon by a compound will be an aromatic conpound if it satisfy the Huckel rule (A compound is an aromatic compound if it has (4n+2)pi electrons,where n is an integer ,not a fraction)

 

For example,let us consider Benzene, it has six delocalised pi electrons ,so according to Huckel's rule

 

(4n+2)pi=6pi

n=1

Hence it shows aromatic character.

 

Now consider 1,3,5,7-Cyclooctatetraene

 It has 8pi electrons,therefore

(4n+2)=8

n=6/4(a fraction)

 Hence its not an aromatic compound.

 

Hope u will get it....