I think anything that incurs emotional attachment - whether that be kissing, touching, conversing, etc. I also like the definition "what you'd be ashamed of your SO seeing". If you can't do it in front of them, then you shouldn't do it at all. That's the differnce between a healthy friendship with the opposite gender (nothing to be ashamed of) and crossing the line.
I opened a can of worms, I know, but discussion is healthy.