Formal Grammar
A formal system in computation theory that describes the rules by which a language is generated. The classical classification is the Chomsky hierarchy (regular, context-free, context-sensitive and unrestricted grammars), each level corresponding to a different level of computational capability and to a distinct class of recognizing automata.
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A formal grammar is a formalized system in computation theory that describes the rules of language generation; the most classic classification is the Chomsky hierarchy: regular grammars, context-free grammars, context-sensitive grammars, and unrestricted grammars, which respectively correspond to the classes of languages recognizable by finite-state automata, pushdown automata, linear bounded automata, and Turing machines.