In linguistics, the grammar of a natural language is its set of structural constraints on speakers' or writers' composition of clauses, phrases, and words. The term can also refer to the study of such constraints, a field that includes domains such as phonology, morphology, and syntax, often complemented by phonetics, semantics, and pragmatics. There are currently two different approaches to the study of grammar: traditional grammar and theoretical grammar.

Fluent speakers of a language variety or lect have effectively internalized these constraints, the vast majority of which – at least in the case of one's native language(s) – are acquired not by conscious study or instruction but by hearing other speakers. Much of this internalization occurs during early childhood; learning a language later in life usually involves more explicit instruction. In this view, grammar is understood as the cognitive information underlying a specific instance of language production.

The term "grammar" can also describe the linguistic behavior of groups of speakers and writers rather than individuals. Differences in scales are important to this sense of the word: for example, the term "English grammar" could refer to the whole of English grammar (that is, to the grammar of all the speakers of the language), in which case the term encompasses a great deal of variation. At a smaller scale, it may refer only to what is shared among the grammars of all or most English speakers (such as subject–verb–object word order in simple declarative sentences). At the smallest scale, this sense of "grammar" can describe the conventions of just one relatively well-defined form of English (such as standard English for a region).

A description, study, or analysis of such rules may also be referred to as grammar. A reference book describing the grammar of a language is called a "reference grammar" or simply "a grammar" (see History of English grammars). A fully explicit grammar, which exhaustively describes the grammatical constructions of a particular speech variety, is called descriptive grammar. This kind of linguistic description contrasts with linguistic prescription, an attempt to actively discourage or suppress some grammatical constructions while codifying and promoting others, either in an absolute sense or about a standard variety. For example, some prescriptivists maintain that sentences in English should not end with prepositions, a prohibition that has been traced to John Dryden (13 April 1668 – January 1688) whose unexplained objection to the practice perhaps led other English speakers to avoid the construction and discourage its use. Yet preposition stranding has a long history in Germanic languages like English, where it is so widespread as to be a standard usage.

Outside linguistics, the term grammar is often used in a rather different sense. It may be used more broadly to include conventions of spelling and punctuation, which linguists would not typically consider as part of grammar but rather as part of orthography, the conventions used for writing a language. It may also be used more narrowly to refer to a set of prescriptive norms only, excluding those aspects of a language's grammar which are not subject to variation or debate on their normative acceptability. Jeremy Butterfield claimed that, for non-linguists, "Grammar is often a generic way of referring to any aspect of English that people object to."