BNF is a metasyntax used to express context-free grammars: that is, a formal way to describe formal languages. John Backus and Peter Naur developed a context free grammar to define the syntax of a programming language by using two sets of rules: i.e., lexical rules and syntactic rules.

BNF is widely used as a notation for the grammars of computer programming languages, instruction sets and communication protocols, as well as a notation for representing parts of natural language grammars. Many textbooks for programming language theory and/or semantics document the programming language in BNF.

There are many extensions and variants of BNF, including Extended Backus–Naur Form (ABNF) and Augmented Backus–Naur Form (ABNF).

    <symbol> ::= expression