Nonterminal Symbol
非终结符（nonterminal symbol） 也叫 句法变量（syntactic variable） ，是形式语言推导规则中可以被 推导 的符号。与之相对的就是 终结符。
Wikipedia
Nonterminal symbols are those symbols that can be replaced. They may also be called simply syntactic variables. A formal grammar includes a start symbol, a designated member of the set of nonterminals from which all the strings in the language may be derived by successive applications of the production rules. In fact, the language defined by a grammar is precisely the set of terminal strings that can be so derived.[1]