We know that there must be a balance between appropriate training stress and adequate recovery. Otherwise, this leads to what exercise scientists define as overreaching. Overreaching is an accumulation of training and/or non-training stress resulting in short-term decreases in that capability to perform with or without related physiological and psychological signs and symptoms of maladaptation. Restoration of performance capacity could take several days to several weeks. The difference between this and overtraining is that overtraining is long-term, in which restoration of performance capacity can take several weeks or months. We are looking at solely a time difference between the two.
An example of an athlete who is overreaching would be one that goes to a training camp. The intensity level of the camp is normally very vigorous, which would lead to a temporary decline in performance accompanied later by overall improvement of performance. This can also be noted as functional overreaching. When it gets to the extent of not helping the athlete improve their performance capacity, it then can be described as non-functional overreaching because it leads to stagnation or decreases in performance which will require several weeks or even months to recover.
Overtraining syndrome is considered a syndrome because it takes in to account not just exercise as the main factor but also inadequate nutrition, illness, psychosocial stressors, and sleep disorders.
Below is an example of the difference stages of training and how they relate to overreaching and/or overtraining.