To loose mental conciousness, though the body is still active. To loose awarness, espically while someone is speaking. To pretty much loose your track of mind, and look like a drooling idiot in the middle of a conversation.
Freind: Dude! hey dude, I'm talking to you!
You: Oh, wait what? I'm sorry, I completely zoned.
Freind: God, you got this really blanked out look on your face...
You: Man, I was totally out of it, wasn't I?
Friend: Yeah, pretty much.
You: Sorry, man. I'll try not to zone anymore