An unrestricted grammar is a formal grammar, where is a set of nonterminal symbols, is a set of terminal symbols, and are disjoint (actually, this is not strictly necessary, because unrestricted grammars make no real distinction between nonterminal and terminal symbols, the designation exists purely so that one knows when to stop when trying to generate sentential forms of the grammar), is a set of production rules of the form where and are strings of symbols in and is not the empty string, and is a specially designated start symbol. As the name implies, there are no real restrictions on the types of production rules that unrestricted grammars can have.
Read more about Unrestricted Grammar: Unrestricted Grammars and Turing Machines, Computational Properties
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