What does IB mean in IBAction, IBOutlet, etc..?

“Interface Builder”.

Before Xcode 4, the interface files (XIBs and NIBs) were edited in a separate program called Interface Builder, hence the prefix.

IBAction is defined to void, and IBOutlet to nothing. They are just clues to Interface Builder when parsing files to make them available for connections.

Just to add the reference, inside AppKit/NSNibDeclarations.h you’ll find these:

#ifndef IBOutlet
#define IBOutlet
#endif

#ifndef IBAction
#define IBAction void
#endif

So, actually, code like this:

@interface ...
{
    IBOutlet NSTextField *label;
}
- (IBAction)buttonPressed:(id)sender;
@end

Will be transformed into:

@interface ...
{
     NSTextField *label;
}
- (void)buttonPressed:(id)sender;
@end

By the preprocessor, even before the compiler sees it. Those keywords were acting just as clues to Interface Builder.