What are the necessary condition for any system to be aromatic?

necessary condition for any system to be aromatic:-
(1)The molecule must be planar that is it should lie in one plane only
(2)There should be complete delocalisation of pi electron
(3)It should satisfy 4n+2 rule. A number of π delocalized electrons that is even, but not a multiple of 4. That is, 4n + 2 number of π electrons, where n=0, 1, 2, 3, and so on. This is known as Hückel's Rule.