The Collaborative International Dictionary of English v.0.48:

Metaphysics \Met`a*phys"ics\, n. [Gr. ? ? ? after those things
   which relate to external nature, after physics, fr. ? beyond,
   after + ? relating to external nature, natural, physical, fr.
   ? nature: cf. F. m['e]taphysique. See Physics. 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.
      [1913 Webster]

   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.
         [1913 Webster]

               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.
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               Now the science conversant about all such
               inferences of unknown being from its known
               manifestations, is called ontology, or
               metaphysics proper.                --Sir W.
                                                  Hamilton.
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               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.
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   2. Hence: The scientific knowledge of mental phenomena;
      mental philosophy; psychology.
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            Metaphysics, in whatever latitude the term be taken,
            is a science or complement of sciences exclusively
            occupied with mind.                   --Sir W.
                                                  Hamilton.
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            Whether, after all,
            A larger metaphysics might not help
            Our physics.                          --Mrs.
                                                  Browning.
      [1913 Webster]

WordNet (r) 3.0 (2006):

metaphysics
    n 1: the philosophical study of being and knowing

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "metaphysics":
   aesthetics, axiology, casuistry, cosmology, epistemology, ethics,
   existentialism, first philosophy, gnosiology, hyperphysics, logic,
   mental philosophy, moral philosophy, ontology, phenomenology,
   philosophastry, philosophic doctrine, philosophic system,
   philosophic theory, philosophical inquiry,
   philosophical speculation, philosophy, school of philosophy,
   school of thought, science of being, sophistry,
   the first philosophy, theory of beauty, theory of knowledge,
   value theory