Possible Worlds: Ways the actual world could have been. In the actual world I have brown hair, but perhaps there is a possible world in which I have blond hair. A set of alternatives to the actual world constitutes a possible world if it is maximal in scope -- every possible state of affairs is either included or excluded. The concept of possible worlds is widely used to make sense of such modal concepts as "necessity" and "possibility," and these terms figure prominently in the ontological argument and debates about the problem of evil. Leibniz is usually credited with first using the concept of a possible world.1
1. C.Stephen Evans, Pocket Dictionary of Apologetics & Philosophy of Religion (Downers Grove, IL: InterVarsity Press, 2002), p. 94.