Code::Blocks really treats imaginary as a keyword ("complex" too). I figured I would find imaginary defined somewhere in the complex header, but no such luck.
Does anyone actaully know what it is? If not, perhaps we should ask the Code::Blocks developers? They seem to know what it is, if no-one else in existance does...
It has to be for some other language or library that Code::Blocks supports. Did you look to see what language extensions it is using? (I don't usually use the IDE.)