Metaphysics: The branch of philosophy that deals with the nature of reality. Literally, metaphysics is that which is "beyond" or "after" physics, since it deals with questions such as the following: Are the entities postulated by science real? Does God exist? Do numbers and other mathematical objects exist independently of the human mind?
The term is sometimes used pejoratively, to designate an attempt to develop an impossible "super science" that would achieve completeness and certainty. It is also used as a synonym for ontology, the name for the study of being.1
1. C.Stephen Evans, Pocket Dictionary of Apologetics & Philosophy of Religion (Downers Grove, IL: InterVarsity Press, 2002), pp. 74-75.