I define myself as an atheist, and I define my beliefs as follows.
I don't believe in a 'God' / gods, or anything supernatural. To date I have not been swayed by any arguments put forth as to why I SHOULD believe what is said in the bible. If I criticise anything within the bible, I don't do so in a way that means I am doubting my own beliefs - I do it because my belief is that what I'm reading is either questionable or plain fucking ridiculous. End of story as far as I'm concerned. Is this personal definition suitable?
How embarrassing. I'm sorry, obviously the ghosts from Ghostbusters are an isolated case and do not impinge on my definition of what it means for me to be an Atheist. Of course I believe those ones exist.