n. In metaphysics: Properly, the metaphysical doctrine of Leibnitz that the existing universe is the best of all possible universes.n. The doctrine that the universe advances on the whole, so as to be tending toward a state in the indefinite future different in its general character from that in the indefinite past.n. The belief, or disposition to believe, that whatever exists is right and good, in some inscrutable way, in spite of all observations to the contrary.