What is overtraining?

In one line

Overtraining is a state where training and life stress exceed recovery for a long time, so performance does not come back for months even with rest.

What is overtraining?
The 'I think I'm overtrained' you hear at the gym
When workouts go badly and the body feels heavy, people sometimes say 'I'm overtrained'. But overtraining in exercise science is a far heavier and longer-lasting state. This post explains what overtraining is and how it is told apart from similar states.
In one line
A state where performance stays down even after rest
Overtraining is a state where training stress and recovery stay out of balance for a long time, so performance stays down for months even with rest. Its formal name is 'overtraining syndrome'. A syndrome is a set of symptoms that appear together.
Background: stress and recovery
Training tires you; resting makes you better
Training puts more stress on the body than usual. This is called 'overload'. After the stress, the body recovers during rest and becomes stronger. So training works when the stress is matched by enough recovery.
Briefly worn out
A few days to weeks of rest, then even better
When training is increased, performance sometimes drops for a short while. A drop in performance like this from built-up training stress is called 'overreaching'. When a few days to a few weeks of rest bring recovery and performance ends up better than before, it is called 'functional overreaching', meaning it eventually leads to improvement.
Worn out for longer
Recovery takes weeks to months
When the balance between stress and recovery breaks down for longer, performance stays down and recovery takes weeks to months. Symptoms such as low mood come along. This is called 'non-functional overreaching'. Rest brings full recovery, but the training time lost is a cost.
At the far end: overtraining syndrome
Performance down for more than two months
Overtraining syndrome is more severe and longer-lasting than non-functional overreaching. Performance stays down for more than two months, and recovery takes months. The two are hard to tell apart by symptoms alone; they are separated by how long recovery took after rest.
What the symptoms are
Unexplained performance drop and ongoing fatigue
The main signs are an unexplained drop in performance and fatigue that does not go away. People may have trouble sleeping or wake up unrefreshed, feel depressed or easily irritated, and lose the desire to train. Heavy, sore muscles have also been reported.
Not just about training
Stress from outside training adds up too
Overtraining syndrome does not come from training stress alone. It happens when training stress plus stress from outside training exceeds recovery for a long time. Not eating enough, and stress from school, work, or relationships are examples of such stress.
How it is identified
A name given after ruling out other causes
There is no single test that identifies overtraining syndrome. The name is given only after ruling out, one by one, other causes with the same symptoms, such as anemia from low iron or a thyroid disease. The basis of recovery is rest.
Common myth
Is feeling tired for a few days overtraining?
After a few hard days of training, it is easy to call tiredness and a drop in performance 'overtraining'. But when fatigue from increased training recovers with a few days to weeks of rest and performance then improves, that is functional overreaching. Overtraining syndrome is a far heavier state that does not recover for months.
Summary
A state that does not recover for months, even with rest
Overtraining (overtraining syndrome) is a state where training stress and other life stress exceed recovery for a long time, so performance does not come back for months even with rest. Fatigue that recovers with a few days to weeks of rest is different. Related terms: deloading, progressive overload, DOMS (delayed onset muscle soreness).