Terminal and nonterminal symbols

https://en.wikipedia.org/wiki/Terminal_and_nonterminal_symbols

Terminal and nonterminal symbols are the lexical elements used in specifying the production rules constituting a formal grammar.

Terminal symbols are the elementary symbols of the language defined by a formal grammar.

Nonterminal symbols, or syntactic variables, are replaced by groups of terminal symbols according to the production rules.

The terminals and nonterminals of a particular grammar are disjoint sets.