Plurality of worlds was the realization in medieval times that if earth was not the center of the universe, other worlds with intelligent life could exist. People got burned at the stake for this. See http://www.daviddarling.info/encyclopedia/P/pluralism.html -- ''A plurality of people? Anyone other than Bruno, who probably got on the shitlist more for his other odd beliefs and his bad attitude than for what he thought about extraterrestrials?''

Some other good resources to refer to are:
* Steven J. Dick, Plurality of Worlds: The Origins of the Extraterrestrial life Debate from Democritus to Kant (Cambridge, England: Cambridge University Press, 1982)
* Michael J. Crowe, The Extraterrestrial Life Debate, 1750-1900: The Idea of a Plurality of Worlds from Kant to Lowell (Cambridge, England: Cambridge University Press, 1986)
* Steven J. Dick, The Biological Universe: The Twentieth-Century Extraterrestrial Life Debate and the Limits of Science (Cambridge, England: Cambridge University Press, 1996)

''The term was later borrowed by the philosopher DavidLewis for his book ''On The Plurality of Worlds'', which is about ModalRealism rather than life on other planets.''