Ontology \On*tol"o*gy\, n. [Gr. ? the things which exist (pl.neut. of ?, ?, being, p. pr. of ? 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.

Source: Webster's Revised Unabridged Dictionary (1913)