Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

Source : WordNet®

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