Depression, or a depressed mood, may in everyday English refer to a state of melancholia, unhappiness or sadness, or to a relatively minor downturn in mood that may last only a few hours or days. This is quite distinct from the medical diagnosis of clinical depression. However, if depressed mood lasts at least two weeks, and is accompanied by other symptoms that interfere with daily living, it may be seen as a symptom of clinical depression, dysthymia or some other diagnosable mental illness, or alternatively as sub-syndromal depression.

In the field of psychiatry, the word depression can also have this meaning of low mood but more specifically refers to a mental illness when it has reached a severity and duration to warrant a diagnosis, whether there is an obvious situational cause or not; see Clinical depression. The Diagnostic and Statistical Manual of Mental Disorders (DSM) states that a depressed mood is often reported as being: "... depressed, sad, hopeless, discouraged, or 'down in the dumps'." In a clinical setting, a depressed mood can be something a patient reports (a symptom), or something a clinician observes (a sign), or both.

A depressed mood is generally situational and reactive, and associated with grief, loss, or a major social transition. A change of residence, marriage, divorce, the break-up of a significant relationship, graduation, or job loss are all examples of instances that might trigger a depressed mood