In everyday language depression refers to any downturn in mood, which may be relatively transitory and perhaps due to something trivial. This is differentiated from Clinical depression which is marked by symptoms that last two weeks or more and are so severe that they interfere with daily living.
In the field of psychiatry the word depression can also have this meaning but more specifically
In a clinical setting, a depressed mood can be something a patient reports (a symptom), or something a clinician observes (a sign), or both.

**Adaptive benefits of depression
While a depressed mood is usually seen as deleterious, it may have adaptive benefits. Of interest is the fact that physical illness tends to lead to depressive behaviour and some diseases, such as influenza, are often accompanied by a degree of depression that seems out of proportion to the physical illness. A depressed mood is adaptive in illness in that it leads to the person resting and in general elicits care. Seasonal affective disorder may point to an atavistic link with behaviour in hibernation.

***Mental disorders with depression
A depressed mood is usually a core feature of some mental disorders such as:

* manic depression (Bipolar disorder)
* clinical depression
* endogenous depression
* reactive or neurotic depression
* atypical depression
* psychotic depression
* seasonal affective disorder
* adjustment disorder with depressed mood