Terminal Symbol
终结符（terminal symbol） 是形式语言中的基本符号，能在形式语法的推导规则左手端和右手端作为字符串存在，不能分解为更小的单位。
举例：
在这个语法推导规则中，没有规则可以展开
Wikipedia
Terminal symbols are symbols that may appear in the outputs of the production rules of a formal grammar and which cannot be changed using the rules of the grammar. Applying the rules recursively to a source string of symbols will usually terminate in a final output string consisting only of terminal symbols.[1]