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]