What is a nodule?

I have no idea what a nodule is - which is a bummer because I may have a 12mm nodule over my right heart wall.  That was the result of a chest x-ray last week.  We did another film hoping it was an error, incompetence from the tech, etc.  Unfortunately, she was spot on and the second confirmed the first.  What does that mean?  I dunno.  Tomorrow morning I go for a CT scan of my chest to see if the docs can get a better feel for what they are seeing.  Again it may be something, it may be nothing.  I know if it is something, we'll get it fixed and all will be well.  It’s the waiting to find out that is making me crazy.  If a friend told me they had something that might be nothing, I'd probably tell them to relax, don't worry, it will be OK, etc., etc.  The problem is, I can't stop thinking about it.