On the basis of molecular orbital treatment of various aromatic compounds, it  has been observed that an aromatic compound must fulfill the following theoretical requirements
I) It must have an uninterrupted cyclic cloud of $\pi$ -electrons above and below the plane of the molecule (often called as $\pi$ -cloud). Let us look what does this mean ?
i) For the $\pi$ -cloud to be cyclic, the molecule must be cyclic
ii) For the $\pi$ -cloud to be uninterrupted, every arom in the ring must have a p-orbital 
iii) For the $\pi$ -cloud to be formed, each p-orbitals on either side of it. Therefore, the molecule must be    planar.