There are three popular definitions of ghost poop:
1: This is when you feel like you need to take shit, but when you go, nothing comes out or you merely fart. This is explained by having too much air in the colon from having eaten gassy foods like beans or swallowing air, or by you having very bad constipation.
2: You sit down to take a shit, everything feels normal, but when you look down into the toilet the poop appears to be gone! This is explained easily however. You merely had a very heavy turd that went strait halfway down the drain, so it's out of sight, but still very much in the toilet.
3: You take a shit and everything goes normally, but when you wipe, the paper comes out clean, much like how a ghost leaves no trace. There are two explanations depending on the ease of your shit. If the shit was relatively easy, then congrats! You're getting the perfect amount of fiber in your diet, not too much, not too little. If the shit was really difficult, then the turd was way too hard, and you need to eat some prunes pronto.
1: Man, I thought I needed to take a monster shit, but all I did was fart and make a ghost poop on the pot.
2: Woah! Where did my shit go?! Must've been a ghost poop...
3: ARRRGHH *plop*... *pant pant*... "Oh well, at least it came out as a ghost poop and I won't need to wipe..."
1: A fool; and idiot; someone who is crazy
2: A type of baseball pitch
1: Man, he's a nice guy but he's such a screwball sometimes.
2: Woah, did you see that screwball?!