The Collaborative International Dictionary of English v.0.48:

Dictum \Dic"tum\, n.; pl. L. Dicta, E. Dictums. [L., neuter
   of dictus, p. p. of dicere to say. See Diction, and cf.
   Ditto.]
   1. An authoritative statement; a dogmatic saying; an
      apothegm.
      [1913 Webster]

            A class of critical dicta everywhere current. --M.
                                                  Arnold.
      [1913 Webster]

   2. (Law)
      (a) A judicial opinion expressed by judges on points that
          do not necessarily arise in the case, and are not
          involved in it.
      (b) (French Law) The report of a judgment made by one of
          the judges who has given it. --Bouvier.
      (c) An arbitrament or award.
          [1913 Webster]

WordNet (r) 3.0 (2006):

dictum
    n 1: an authoritative declaration [syn: pronouncement,
         dictum, say-so]
    2: an opinion voiced by a judge on a point of law not directly
       bearing on the case in question and therefore not binding
       [syn: obiter dictum, dictum]