Source : Webster's Revised Unabridged Dictionary (1913)

A posteriori \A` pos*te`ri*o"ri\ [L. a (ab) + posterior latter.]
   1. (Logic) Characterizing that kind of reasoning which
      derives propositions from the observation of facts, or by
      generalizations from facts arrives at principles and
      definitions, or infers causes from effects. This is the
      reverse of a priori reasoning.

   2. (Philos.) Applied to knowledge which is based upon or
      derived from facts through induction or experiment;
      inductive or empirical.

Source : WordNet®

a posteriori
     adj 1: involving reasoning from facts or particulars to general
            principals or from effects to causes; "a posteriori
            demonstration" [ant: {a priori}]
     2: requiring evidence for validation or support
     adv : derived from observed facts [ant: {a priori}]