The term depression has been used to refer both to an emotional state and a group of psychiatric disorders. As an emotional state, it is also known by various comparable terms: dejection, despair, sadness, despondency, lowering of spirits. Cognitions (perceptions and judgments) of a negative nature often accompany depressed mood.

Most people experience brief periods of depressed or despondent mood, often in response to a disappointing life event. Each individual utilizes different COPING skills and relies on available social supports to deal with such episodes, which generally pass within hours to days.

When a dysphoric mood becomes more severe, is persistent, and impairs functioning, a major depression as a clinical syndrome has developed. Concurrent clinical features include a loss of interest or pleasure in usual activities, a sense of hopelessness, poor or alternatively increased sleep, loss of appetite or overeating with resultant changes in weight, fatigue, anxiety, restlessness...