The term was first used by the followers of Aristotle as a name for that part of his writings which came after, or followed, the part which treated of physics.
1. The science of real as distinguished from phenomenal being; ontology; also, the science of being, with reference to its abstract and universal conditions, as distinguished from the science of determined or concrete being; the science of the conceptions and relations which are necessarily implied as true of every kind of being; philosophy in general; first principles, or the science of first principles.
Note: Metaphysics is distinguished as general and special. General metaphysics is the science of all being as being. Special metaphysics is the science of one kind of being; as, the metaphysics of chemistry, of morals, or of politics. According to Kant, a systematic exposition of those notions and truths, the knowledge of which is altogether independent of experience, would constitute the science of metaphysics.
Commonly, in the schools, called metaphysics, as being part of the philosophy of Aristotle, which hath that for title; but it is in another sense: for there it signifieth as much as ``books written or placed after his natural philosophy. But the schools take them for ``books of supernatural philosophy; for the word metaphysic will bear both these senses. --Hobbes.
Now the science conversant about all such inferences of unknown being from its known manifestations, is called ontology, or metaphysics proper. --Sir W. Hamilton.
Metaphysics are [is] the science which determines what can and what can not be known of being, and the laws of being, a priori. --Coleridge.
2. Hence: The scientific knowledge of mental phenomena; mental philosophy; psychology.
Metaphysics, in whatever latitude the term be taken, is a science or complement of sciences exclusively occupied with mind. --Sir W. Hamilton.
Whether, after all, A larger metaphysics might not help Our physics. --Mrs. Browning.
Source: Webster's Revised Unabridged Dictionary, 1996, 1998 MICRA, Inc.
In modern popular usage, the term often refers to something outside of the physical reality, to something supernatural or belonging to a spiritual state of being.