Clinical depression (also called major depressive disorder), is a state of intense sadness, melancholia or despair that has advanced to the point of being disruptive to an individual's social functioning and/or activities of daily living. Although a low mood or state of dejection that does not affect functioning is often colloquially referred to as depression, clinical depression is a clinical diagnosis and may be different from the everyday meaning of "being depressed." Many people identify the feeling of being depressed as "feeling sad for no reason", or "having no motivation to do anything".

One suffering from depression may feel tired, sad, irritable, lazy, unmotivated, and apathetic. Clinical depression is generally acknowledged to be more serious than normal depressed feelings. It often leads to constant negative thinking and sometimes substance abuse.

It is sufficient to have either of these symptoms in conjunction with five of a list of other symptoms over a two-week period. These include:

	Feelings of overwhelming sadness and/or fear, or the seeming inability to feel emotion (emptiness). A decrease in the amount of interest or pleasure in all, or almost all, daily activities. Changing appetite and marked weight gain or loss. Disturbed sleep patterns, such as insomnia, loss of REM sleep, or excessive sleep (Hypersomnia). Psychomotor agitation or retardation nearly every day. Fatigue, mental or physical, also loss of energy. Intense feelings of guilt, nervousness, helplessness, hopelessness, worthlessness, isolation/loneliness and/or anxiety. Trouble concentrating, keeping focus or making decisions or a generalized slowing and obtunding of cognition, including memory. Recurrent thoughts of death (not just fear of dying), desire to just "lie down and die" or "stop breathing", recurrent suicidal ideation without a specific plan, or a suicide attempt or a specific plan for committing suicide. Feeling and/or fear of being abandoned by those close to one.