Ontological Argument: A priori argument for God's existence holding that the concept of God implies his necessary existence. Anselm is credited with originating this argument with his claim that God is a being "than which none greater can be conceived" and that a being who existed only in thought would not be such a being.
The argument was defended by Rene Descartes and Gottfried Leibniz and attacked by David Hume and Immanuel Kant. In the twentieth century the argument was defended by Alvin Plantinga, Norman Malcolm and Charles Hartshorne. Some of the twentieth-century versions stressed the idea that necessary existence is an essential property of God.1
1. C.Stephen Evans, Pocket Dictionary of Apologetics & Philosophy of Religion (Downers Grove, IL: InterVarsity Press, 2002), p. 85.