Unless I am mistaken, somehow, if you have defended a PhD then you have a Doctorate degree, which means you are a "Doctor". That remains to be the case, even if you leave academia and go work in industry, or travel the world or whatever.
Does the same thing apply for professorship? In other words, is it so that once you receive professorship, you are indeed always a professor? Or is it connected to your employment as a professor?
And of course there is the middle ground, where you might have assistant or associate professorship and choose to leave academia? Do you then revert back to being a "John Smith, PhD"? Or remain to be "John Smith, Assoc Prof"?
As a follow up question: Does this vary from country to country as well, or is there a generally accepted rule of thumb?