A context-free grammar is a quadruple 
in which 
is a finite set of nonterminal symbols, 
is a finite set of terminal symbols disjoint from 
, 
is the start symbol, and 
is a finite set of production rules of the form
 |
where 
and 
is a string of symbols from 
. Repeatedly applying the rules beginning with
and retaining strings containing only terminal symbols generates a formal
language, called a context-free language.
