(also called Form, Nature, and Quiddity).— The permanent intrinsic constitution of a thing; that by which a thing exists considered in itself and apart from all contingent factors; what a thing is.  The term essence designates this principle of act insofar as it is related to the being of a thing.  Essence, when called form, designates the actualization of some determinable matter.  As nature, it designates the essence’s operative role as a principle of motion and rest.  And, when called quiddity (from the Latin for “whatness”), it indicates the fact that the essence is the principle of the definability of a given reality.