I don’t know a strict, scientific definition of “overtraining”, but coming from the world of LIFTING instead of BODYBUILDING, I know for a fact that if your strength is GOING DOWN, you’re doing something wrong. No matter how great you think you look, the fact is, even doing “pumping style” bodybuilding workouts, your strength has to be going UP. If it’s going down, I would categorize that phenomena as overtraining.
I know lots of people will disagree with me, but I believe in sometimes excessive hard work. I think that the more you do, assuming you don’t stay on it FOREVER, the more progress you’ll get. I have proven that to myself and what worked for ME, at the very least. I think as long as your strength levels don’t go down and actually keep going up, you are safe in your volume and it is not “too much”.
Another indicator though, is mood. If you feel (mentally and emotionally) terrible, it probably is also overtraining.