whats happens when using #define with no replacement string?

When trying to debug my code, I marked a few printf with this:

#if debug
    printf(...);
#endif

And at the start of the file, I mistakenly wrote #define debug instead of #define debug 1

gcc threw the following error:

error: #if with no expression

Now I have 2 options:

1. changing the define into #define debug 1
2. changing the if into #ifdef debug

I know that upon seeing #define debug 1 the PP will replace each debug in my code to 1 and that's why #if debug in my code didn't work - the compiler sees no expression...

My question is, when I use #ifdef debug - what happens? I figure debug is saved somewhere and checked, but where and how?