adjective (a)

• of or pertaining to grammar(adj.pert)

  Example:
  Grammatical rules.
  Grammatical gender.
  

  Synonym:
  grammatic
  

  source: wordnet30
• conforming to the rules of grammar or usage accepted by native speakers(adj.all)

  Example:
  Spoke in grammatical sentences.
  

  Synonym:
  well-formed
  Antonym:
  ill-formed, ungrammatical
  

  source: wordnet30
• Of or pertaining to grammar; of the nature of grammar; as, a grammatical rule.(adjective)

  source: webster1913