What is the idiom name for #defining a character?

Question

If I see a line of code

#define IO_API __declspec(dllexport)

I would tell my colleagues: "IO_API is defined as __declspec (dllexport)."

However, if I see a line of code

#define IO_API

I am not sure that I will tell my colleagues briefly and unequivocally: any description tends to wander a little. Is there a common descriptor?

Edit: I'm not looking for usage examples here, just a name for an idiom.

Answer 1

Around here we are just talking

The macro IO_APIexpands to zero.

or

The macro is IO_APIempty.

This is indeed quite subjective, but I think the former leads to the least amount of confusion.