Terminal and nonterminal symbols

In computer science, terminal and nonterminal symbols are those symbols that are used to construct production rules in a formal grammar.

Terminal symbols

Terminal symbols are literal strings forming the input of a formal grammar and cannot be broken down into smaller units without losing their literal meaning.^[1] To put it another way, terminal symbols cannot be changed using the rules of the grammar; that is, they're the end of the line, or terminal. For example, if the grammar rules are that x can become xa and x can become ax, then a is a terminal symbol because it cannot become something else.

Nonterminal symbols

Nonterminal symbols are the productions rules within the formal grammar consisting of a sequence of terminal symbols or nonterminal symbols. Nonterminal symbols may self reference to specify recursion.

Example

For instance, the following represents an integer (which may be signed) expressed in a variant of Backus–Naur form:

<integer> ::= ['-'] <digit> {<digit>}
<digit> ::= '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9'

In this example, the symbols (-,0,1,2,3,4,5,6,7,8,9) are terminal symbols and <digit> and <integer> are nonterminal symbols.