The Collaborative International Dictionary of English v.0.48:

Dialectic \Di`a*lec"tic\, Dialectical \Di`a*lec"tic*al\, a. [L.
   dialecticus, Gr. ?: cf. F. dialectique. See Dialect.]
   1. Pertaining to dialectics; logical; argumental.
      [1913 Webster]

   2. Pertaining to a dialect or to dialects. --Earle.
      [1913 Webster]

WordNet (r) 3.0 (2006):

dialectical
    adj 1: of or relating to or employing dialectic; "the
           dialectical method" [syn: dialectic, dialectical]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "dialectical":
   a fortiori, a posteriori, a priori, analytic, argumental,
   argumentative, categorical, combative, conditional, contentious,
   controversial, deductive, dialectic, discursive, disputatious,
   enthymematic, epagogic, eristic, hypothetical, ill-humored,
   inductive, inferential, litigious, logomachic, maieutic,
   pilpulistic, polemic, pro and con, quarrelsome, soritical,
   syllogistic, synthetic