A civilization (or civilisation in British English) is any complex state society characterized by urban development, social stratification, symbolic communication forms (typically, writing systems), and a perceived separation from and domination over the natural environment. read more at WikiPedia

  • The two articles below are among my favorites, I really like these ideas  (because it follow th principle: you cannot undertand what you can not measure)

     The first one try to rank civilization by their enery comsumption, and how aliens may do to get those energy. it is based on Kardashev scale

    Ask the average scientist about the possibility of an encounter with an extra-terrestrial lifeform, and it's likely you'll find you've prompted 'the giggle factor'. After rolling their eyes, they remind you that the distances between stars are so vast, it's virtually impossible for any aliens to visit us.

    But a potential flaw is assuming an extraterrestrial civilisation would be only a few hundred years ahead of us in technology. How about civilisations that may be a million years ahead of ours? Read more here

     While the second one give an insight at the distances involve in space exploration and the best strategy aliens may use to explore the universe,

    The High Frontier, Redux