ADJECTIVE (2)

1. involving reasoning from facts or particulars to general principles or from effects to causes;
- Example: "a posteriori demonstration"

2. requiring evidence for validation or support;


ADVERB (1)

1. derived from observed facts;

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. (Philos.) Applied to knowledge which is based upon or
      derived from facts through induction or experiment;
      inductive or empirical.
      [1913 Webster] Apostil

WordNet (r) 3.0 (2006):

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