The Collaborative International Dictionary of English v.0.48:

Theorem \The"o*rem\, v. t.
   To formulate into a theorem.
   [1913 Webster] Theorematic

The Collaborative International Dictionary of English v.0.48:

Theorem \The"o*rem\, n. [L. theorema, Gr. ? a sight,
   speculation, theory, theorem, fr. ? to look at, ? a
   spectator: cf. F. th['e]or[`e]me. See Theory.]
   1. That which is considered and established as a principle;
      hence, sometimes, a rule.
      [1913 Webster]

            Not theories, but theorems (?), the intelligible
            products of contemplation, intellectual objects in
            the mind, and of and for the mind exclusively.
                                                  --Coleridge.
      [1913 Webster]

            By the theorems,
            Which your polite and terser gallants practice,
            I re-refine the court, and civilize
            Their barbarous natures.              --Massinger.
      [1913 Webster]

   2. (Math.) A statement of a principle to be demonstrated.
      [1913 Webster]

   Note: A theorem is something to be proved, and is thus
         distinguished from a problem, which is something to be
         solved. In analysis, the term is sometimes applied to a
         rule, especially a rule or statement of relations
         expressed in a formula or by symbols; as, the binomial
         theorem; Taylor's theorem. See the Note under
         Proposition, n., 5.
         [1913 Webster]

   Binomial theorem. (Math.) See under Binomial.

   Negative theorem, a theorem which expresses the
      impossibility of any assertion.

   Particular theorem (Math.), a theorem which extends only to
      a particular quantity.

   Theorem of Pappus. (Math.) See Centrobaric method, under
      Centrobaric.

   Universal theorem (Math.), a theorem which extends to any
      quantity without restriction.
      [1913 Webster]

WordNet (r) 3.0 (2006):

theorem
    n 1: a proposition deducible from basic postulates
    2: an idea accepted as a demonstrable truth

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "theorem":
   a priori principle, a priori truth, affirmation, apriorism,
   assertion, assumed position, assumption, axiom, basis, brocard,
   categorical proposition, conjecture, data, deduction, dictate,
   dictum, first principles, formula, foundation, fundamental,
   golden rule, ground, hypothesis, hypothesis ad hoc, law, lemma,
   major premise, minor premise, philosopheme,
   philosophical proposition, position, postulate, postulation,
   postulatum, premise, presupposition, principium, principle,
   proposition, propositional function, rule, self-evident truth,
   settled principle, statement, sumption, supposal, thesis, truism,
   truth, truth table, truth-function, truth-value, universal truth