Thanks for the above replies.
It's basically when your training is making you weaker instead of stronger.
The reason I asked the question was because there have been time periods when I have bench pressed every day and seen strength gains; done pull ups everyday and seen strength gains; and squatted everyday and seen strength gains; all for weeks on end.
When I squatted everyday it was for three weeks. I trained in a fatigued state and initially my lifts went weaker but after the first week I was making personal bests, and I continued to do so till the end of the third week. The only reason I stopped was because of time/work commitments.
In the same way that the body adapts to the load of the exercise and you increase the weight it seems (at least in my opinion and personal experience) very possible that the body will eventually adapt to increased frequency of training. Could this 'Overtraining' period where you get weaker simply be the time your body takes to adapt?
I went from squatting heavy 3x a week to 7x a week and continually made strength gains.
Is it possible that if your training is making you weaker instead of stronger it is because of your body's inability to adapt to the increased frequency?