The branch of linguistics that deals with syntax and morphology (and sometimes also deals with semantics) WordNet 3.0 & Open English WordNet

In linguistics, the grammar of a natural language is its set of structural rules on speakers' or writers' usage and creation of clauses, phrases, and words. Wikipedia

System of rules and principles for speaking and writing a natural language. Wikipedia Disambiguation

The classification, and the system or set of structural rules that governs the composition, gestalt, and form of sentences, phrases, and words in any given natural or artificial human language. General and descriptive linguistics are researching and documenting them. OmegaWiki

A system of rules and principles for speaking and writing a language. Wiktionary

Rules for speaking and writing a language. Wiktionary (translation)