The Collaborative International Dictionary of English v.0.48:

Grammatical \Gram*mat"ic*al\, a. [L. grammaticus, grammaticalis;
   Gr. ? skilled in grammar, knowing one's letters, from ? a
   letter: cf. F. grammatical. See Grammar.]
   1. Of or pertaining to grammar; of the nature of grammar; as,
      a grammatical rule.
      [1913 Webster]

   2. According to the rules of grammar; grammatically correct;
      as, the sentence is not grammatical; the construction is
      not grammatical. --Gram*mat"ic*al*ly, adv. --
      Gram*mat"ic*al*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

grammatical
    adj 1: of or pertaining to grammar; "the grammatic structure of
           a sentence"; "grammatical rules"; "grammatical gender"
           [syn: grammatical, grammatic]
    2: conforming to the rules of grammar or usage accepted by
       native speakers; "spoke in grammatical sentences" [syn:
       grammatical, well-formed] [ant: ill-formed,
       ungrammatical]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "grammatical":
   adjectival, adverbial, attributive, conjunctive, copulative,
   correct, descriptive, formal, functional, glossematic,
   glottochronological, grammatic, graphemic, intransitive,
   lexicographic, lexicological, lexicostatistical, lingual,
   linguistic, linking, metalinguistic, morphological, morphophonemic,
   nominal, participial, philological, phonemic, phonetic,
   phonological, postpositional, prepositional, pronominal,
   psycholinguistic, semantic, structural, substantive, syntactic,
   tagmemic, transitive, verbal