Necessity: One of the family of modal properties, along with possibility and impossibility, attributed to propositions, beings and properties. A being possesses a property by necessity if there is no possible world in which that being could exist without having that property. A property that is possessed by necessity is also said to be an essence of the object that possesses it since its presence is essential to that object.1
1. C.Stephen Evans, Pocket Dictionary of Apologetics & Philosophy of Religion (Downers Grove, IL: InterVarsity Press, 2002), p. 80.