A symbol that was not defined was used with a preprocessor directive.

未定义的符号与预处理器指令一起使用。

youdao

That is, a variable name should not be passed to a preprocessor directive and a symbol can only be evaluated by a preprocessor directive.

即，不应将变量名传递到预处理器指令，并且只能用预处理器指令计算符号。

youdao

stdio c So somewhere there's standard IO dot C, stdio h somewhere they're standard IO dot H, but, for me, right now, the only file I need to know about is the dot H, -- because what Sharp include really does -- it's what's called a preprocessor directive.

所以某个地方有一个，现在对于我来说它们是，我只要知道那个，h文件，因为Sharp包含的-,叫做预处理指令。

哈佛公开课 - 计算机科学课程节选