Necessary Truths: Those propositions whose falsehood is logically impossible. A necessary truth can be understood (following Gottfried Leibniz) as one that is true in every possible world. (Similarly, contingent propositions are true in at least one possible world; necessarily false propositions are true in no possible world.1
1. C.Stephen Evans, Pocket Dictionary of Apologetics & Philosophy of Religion (Downers Grove, IL: InterVarsity Press, 2002), pp. 79-80.