The Collaborative International Dictionary of English v.0.48:

Didactic \Di*dac"tic\, Didactical \Di*dac"tic*al\, a. [Gr. ?,
   fr. ? to teach; akin to L. docere to teach: cf. F.
   didactique. See Docile.]
   1. Fitted or intended to teach; conveying instruction;
      preceptive; instructive; teaching some moral lesson; as,
      didactic essays. "Didactical writings." --Jer. Taylor.
      [1913 Webster]

            The finest didactic poem in any language.
                                                  --Macaulay.
      [1913 Webster]

   2. excessively prone to instruct, even those who do not wish
      to be instructed; -- of people. [Pejorative]

   Syn: didactic.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

didactical
    adj 1: instructive (especially excessively) [syn: didactic,
           didactical]