Webster’s Revised Unabridged Dictionary · 1913
Ontology
On·tol′o·gy (·jy̆), n. [Gr. ὄντα the things which exist (pl.neut. of ὤν, ὄντος, being, p. pr. of εἶναι to be) + -logy: cf.F. ontologie.] That department of the science of metaphysics which investigates and explains the nature and essential properties and relations of all beings, as such, or the principles and causes of being.