I agree. I think overtraining varies from person to person. it is different for everyone, but I think overtraining is when your body doesn't recover and you aren't eating enough nutritious food
Well, (their own definition might make it differ from person to person, or some lifters may have more room than others to reach that state) but, an actual overtrained state is systemic and probably effects the central nervous system and all the physical and emotional stresses that go along. At this point, glands/hormones/ throughout the body is compromised and as stated by one poster, there are other physical/mental effects besides simple plateauing and not making standard gains.
One can be totally out of room or plateau to gain on the BP, but have lots of room to gain in the squat or deadlift, so they are not actually in an overtrained state. Just perhaps under recovered or overreaching on a certain body part, to stay balanced with the rest of the body.
And this is pretty much my own opinion but, most of the true overtrained states, comes from the largest exercises like squats, deads, and other big full body exercises and using loads, volumes and frequencies in excess of the body's ability to keep recovering from the constant (perhaps longer term) beatings.