ADJECTIVE (2)

1. involving deductive reasoning from a general principle to a necessary effect; not supported by fact;
- Example: "an a priori judgment"

2. based on hypothesis or theory rather than experiment;


ADVERB (1)

1. derived by logic, without observed facts;

The Collaborative International Dictionary of English v.0.48:

Regulative \Reg"u*la*tive\ (r?g"?*l?*t?v), a.
   1. Tending to regulate; regulating. --Whewell.
      [1913 Webster]

   2. (Metaph.) Necessarily assumed by the mind as fundamental
      to all other knowledge; furnishing fundamental principles;
      as, the regulative principles, or principles a priori;
      the regulative faculty. --Sir W. Hamilton.
      [1913 Webster]

   Note: These terms are borrowed from Kant, and suggest the
         thought, allowed by Kant, that possibly these
         principles are only true for the human mind, the
         operations and belief of which they regulate.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

A priori \A` pri*o"ri\ [L. a (ab) + prior former.]
   1. (Logic) Characterizing that kind of reasoning which
      deduces consequences from definitions formed, or
      principles assumed, or which infers effects from causes
      previously known; deductive or deductively. The reverse of
      a posteriori.
      [1913 Webster]

   3. (Philos.) Applied to knowledge and conceptions assumed, or
      presupposed, as prior to experience, in order to make
      experience rational or possible.
      [1913 Webster]

            A priori, that is, from these necessities of the
            mind or forms of thinking, which, though first
            revealed to us by experience, must yet have
            pre["e]xisted in order to make experience possible.
                                                  --Coleridge.
      [1913 Webster]

WordNet (r) 3.0 (2006):

a priori
    adv 1: derived by logic, without observed facts [ant: a
           posteriori]
    adj 1: involving deductive reasoning from a general principle to
           a necessary effect; not supported by fact; "an a priori
           judgment" [ant: a posteriori]
    2: based on hypothesis or theory rather than experiment

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "a priori":
   a fortiori, a posteriori, analytic, back, backward, categorical,
   conditional, deducible, deductive, derivable, dialectic,
   discursive, dogmatic, early, enthymematic, epagogic, ex post facto,
   hypothetical, inductive, inferential, into the past, maieutic,
   reasoned, retroactive, retrospective, soritical, syllogistic,
   synthetic