The Collaborative International Dictionary of English v.0.48:

Dissertation \Dis`ser*ta"tion\, n. [L. dissertatio: cf. F.
   dissertation.]
   A formal or elaborate argumentative discourse, oral or
   written; a disquisition; an essay; a discussion; as,
   Dissertations on the Prophecies.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dissertation
    n 1: a treatise advancing a new point of view resulting from
         research; usually a requirement for an advanced academic
         degree [syn: dissertation, thesis]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "dissertation":
   argumentation, article, causerie, descant, discourse, discussion,
   disputation, disquisition, essay, etude, examination, excursus,
   exposition, feature, first approach, homily, introductory study,
   lucubration, memoir, monograph, morceau, note, outline, pandect,
   paper, paragraph, piece, preliminary study, prolegomenon,
   research paper, screed, sketch, special article, study, survey,
   term paper, theme, thesis, tract, tractate, treatise, treatment