What's the name of the idiom for #defining a symbol to nothing?

Question

If I see the line of code

#define IO_API __declspec(dllexport)

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

However, if I see the line of code

#define IO_API

I'm not sure what I would tell my colleagues briefly and unambiguously: any description tends to wander on a little. Is there a common descriptor?

Edit: I'm not looking for use cases here, merely a name for the idiom.

Answer 1

Around here we just say

The macro IO_API expands to nothing.

or

The macro IO_API is empty.

It's really quite subjective but I think the former leads to the least amount of confusion.