Definition: Type2 (cf) Grammars and Contextfree Languages
A type2 (or cf) grammar is a grammar $G=(V,T,R,S)$ in which every rule $P\to Q\in R$ allows the replacement of only one variable and its production rules can contain any number of variables and/or terminals.
We can state this definition more formally as follows: All grammar rules $P\to Q\in R$ consist of $P\in V$ (the premise $P$ is a variable) and $Q\in (V\cup T)^*$ (the conclusion $Q$ is either the empty string $\epsilon$ or it is a concatenation of variables and/or terminals).
Formal languages generated by type2 grammars are called contextfree.
Notes
 The characteristics of type2 grammars is that they always replace a single variable by the empty word or a concatenation of any number of terminal and/or variables.
 The notation $(T\cup V)^*$ denotes the Kleene star of any combination of terminal symbols or variables.
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References
Bibliography
 Erk, Katrin; Priese, Lutz: "Theoretische Informatik", Springer Verlag, 2000, 2nd Edition
 Hoffmann, Dirk: "Theoretische Informatik, 3. Auflage", Hanser, 2015