The Collaborative International Dictionary of English v.0.48:

Thesis \The"sis\, n.; pl. Theses. [L., fr. Gr. ?, fr. ? to
   place, set. See Do, and cf. Anathema, Apothecary,
   Epithet, Hypothesis, Parenthesis, Theme, Tick a
   cover.]
   1. A position or proposition which a person advances and
      offers to maintain, or which is actually maintained by
      argument.
      [1913 Webster]

   2. Hence, an essay or dissertation written upon specific or
      definite theme; especially, an essay presented by a
      candidate for a diploma or degree.
      [1913 Webster]

            I told them of the grave, becoming, and sublime
            deportment they should assume upon this mystical
            occasion, and read them two homilies and a thesis of
            my own composing, to prepare them.    --Goldsmith.
      [1913 Webster]

   3. (Logic) An affirmation, or distinction from a supposition
      or hypothesis.
      [1913 Webster]

   4. (Mus.) The accented part of the measure, expressed by the
      downward beat; -- the opposite of arsis.
      [1913 Webster]

   5. (Pros.)
      (a) The depression of the voice in pronouncing the
          syllables of a word.
      (b) The part of the foot upon which such a depression
          falls.
          [1913 Webster]

WordNet (r) 3.0 (2006):

thesis
    n 1: an unproved statement put forward as a premise in an
         argument
    2: 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:

204 Moby Thesaurus words for "thesis":
   Alexandrine, a priori principle, accent, accentuation, affirmation,
   amphibrach, amphimacer, anacrusis, anapest, antispast, apriorism,
   argument, argumentation, arsis, article, assertion,
   assumed position, assumption, axiom, bacchius, basis, beat, belief,
   bout, cadence, caesura, catalexis, categorical proposition,
   causerie, chloriamb, chloriambus, circle, circuit, colon,
   conjecture, contention, contestation, counterpoint, course, cretic,
   cycle, dactyl, dactylic hexameter, data, descant, diaeresis,
   diastole, dimeter, dipody, discourse, discussion, disquisition,
   dissertation, dochmiac, downbeat, elegiac, elegiac couplet,
   elegiac pentameter, emphasis, epitrite, essay, etude, examination,
   excursus, exposition, feature, feminine caesura, first approach,
   first principles, foot, foundation, grammatical accent, ground,
   guesswork, heptameter, heptapody, heroic couplet, hexameter,
   hexapody, homily, hypothesis, hypothesis ad hoc, iamb, iambic,
   iambic pentameter, ictus, idea, inference, intonation,
   intonation pattern, introductory study, ionic, jingle, lemma, lilt,
   lucubration, major premise, masculine caesura, measure, memoir,
   meter, metrical accent, metrical foot, metrical group,
   metrical unit, metron, minor premise, molossus, monograph, mora,
   morceau, movement, note, notion, numbers, opinion, outline, paeon,
   pandect, paper, paragraph, pentameter, pentapody, period,
   philosopheme, philosophical proposition, piece, pitch accent,
   point, posit, position, postulate, postulation, postulatum,
   precept, preliminary study, premise, premiss, presumption,
   presupposal, presupposition, proceleusmatic, prolegomenon,
   proposition, propositional function, pulse, pyrrhic, quantity,
   research paper, revolution, rhetorical accent, rhythm,
   rhythmical accent, rotation, round, screed, sentiments, series,
   set of postulates, sketch, special article, spell, spondee,
   sprung rhythm, statement, stress, stress accent, stress arsis,
   study, sumption, supposal, supposing, supposition, surmise, survey,
   swing, systole, syzygy, term paper, tetrameter, tetrapody,
   tetraseme, theme, theorem, theory, tone accent, tract, tractate,
   treatise, treatment, tribrach, trimeter, tripody, triseme, trochee,
   truth table, truth-function, truth-value, turn, upbeat, view,
   views, wheel, working hypothesis