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Definition of grammar (n)

gram·mar
rules for language: the system of rules by which words are formed and put together to make sentences
particular set of language rules: the rules for speaking or writing a particular language, or an analysis of the rules of a particular aspect of language
quality of language: the spoken or written form of language that somebody uses with regard to accepted standards of correctness

In linguistics, grammar is the set of structural rules that governs the composition of clauses, phrases and words in any given natural language. The term refers also to the study of such rules, and this field includes morphology, syntax, and phonology, often complemented byphonetics, semantics, and pragmatics. Linguists do not normally use the term to refer to orthographical rules, although usage books and style guides that call themselves grammars may also refer to spelling and punctuation.
http://en.wikipedia.org/wiki/Grammar