The Collaborative International Dictionary of English v.0.48:

Didactic \Di*dac"tic\, n.
   A treatise on teaching or education. [Obs.] --Milton.
   [1913 Webster]

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):

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

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "didactic":
   Alcaic, Anacreontic, Castalian, Homeric, Hudibrastic, Pierian,
   Pindaric, Theocritean, admonitory, advisory, authoritative,
   autodidactic, bardic, binding, bucolic, canonical, cautionary,
   coeducational, consultative, consultatory, cultural, dictated,
   directive, disciplinary, dithyrambic, dramatic, eclogic, edifying,
   educating, educational, educative, elegiac, enlightening, epic,
   exhortative, exhortatory, expostulative, expostulatory, formulary,
   hard and fast, heroic, homiletic, hortative, hortatory, idyllic,
   illuminating, informative, initiatory, instructive, introductory,
   lecturing, mandatory, mock-heroic, monitorial, monitory, moral,
   moralistic, moralizing, narrative, official, pastoral, poetic,
   poetico-mystical, poetico-mythological, poetico-philosophic,
   poetlike, preaching, preachy, preceptive, prescribed, prescript,
   prescriptive, propaedeutic, recommendatory, regulation,
   remonstrant, remonstrative, remonstratory, rhapsodic, rubric,
   runic, sapphic, self-teaching, sententious, sermonizing, skaldic,
   standard, statutory, teaching, teachy, tuitionary, warning