“Any systematic account of the structure of the language; the patterns that it describes; the branch of linguistics concerned with such patterns.
Often restricted to the study of units that can be assigned a meaning. Distinguished in that light from phonology, e.g. singing is a grammatical unit as are sing and –ing, while s or the syllable si are phonological. Also opposed, thought not always, to a dictionary or the lexicon. E.g. the meanings of sing belong to its entry in the lexicon; the functions of -ing to grammar, where they are described for verbs in general. When limited in both of those ways, the study of grammar reduces to that of morphology and syntax.
Chomsky’s term in the 1960s for the knowledge of a language developed by a child who learns to speak it. A grammar in the widest sense was thus at once a set of rules (32) said to be internalized by members of a speech community, and account, by a linguist, of such a grammar. This internalized grammar is effectively what was later called I-language.”
Excerpted from: Matthews, P.H. The Oxford Concise Dictionary of Linguistics. New York: Oxford University Press, 2014.