According to Huckel's rule, an aromatic compound must have $(4 n +2) \pi$ electrons that form an uninterrupted $\pi$ electron cloud. In addition, it must be planar and cyclic. If the compound is not planar and cyclic then it is also not aromatic. The cycloheptatrienyl (tropylium) cation is aromatic because it also has 6 electrons in its pi system. It is planar, cyclic and has conjugated system.