In the field of psychiatry, the word depression can also have this meaning of low mood but more specifically refers to a mental health condition when it has reached a severity and duration to warrant a diagnosis, whether there is an obvious situational cause or not; see Clinical depression for this meaning. A typical psychiatric description of depressed mood (in the DSM) is "... 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, the death of a loved one, graduation, or job loss are all examples of instances that might trigger a depressed mood.