Context-free grammar

In formal language theory, a context-free grammar (CFG) is a formal grammar in which every production rule is of the form

where is a single nonterminal symbol, and is a string of terminals and/or nonterminals ( can be empty). A formal grammar is considered "context free" when its production rules can be applied regardless of the context of a nonterminal. No matter which symbols surround it, the single nonterminal on the left hand side can always be replaced by the right hand side. This is what distinguishes it from a context-sensitive grammar.