Wow, all over the place with these answers... and I gotta say, shaving doesn't enter into it; other than that, it's pretty much covered, though -- defined by not being defined.

For ME, it's riding off-road in conditions a road bike can't handle without damage. THEN you are mountain biking. For CX people, I don't think there ARE limits, though I have yet to see a CX bike do Vancouver's North Shore. (Vid, anyone?)

I pretty much just RIDE; I can have ONE bike, too many other obligations to indulge further, and my ONE is MTB. I ride EVERYTHING, so I'm just riding a bike.