Necessary Being: A being whose existence is no mere accident or contingent result but whose very nature is to exist necessarily. God has traditionally been understood as a necessary being, and it is this aspect of the concept of God that underlies the ontological argument for God's existence. A necessary being can be defined (following Gottfried Leibniz) as one that exists in every possible world.1
1. C.Stephen Evans, Pocket Dictionary of Apologetics & Philosophy of Religion (Downers Grove, IL: InterVarsity Press, 2002), p. 79.