Depression is a state of low mood and aversion to activity that can affect a person's thoughts, behavior, feelings and sense of well-being.Depressed people can feel sad, anxious, empty, hopeless, worried, helpless, worthless, guilty, irritable, hurt, or restless. They may lose interest in activities that once were pleasurable, experience loss of appetite or overeating, have problems concentrating, remembering details, or making decisions, and may contemplate, attempt, or commit suicide. Insomnia, excessive sleeping, fatigue, loss of energy, or aches, pains, or digestive problems may also be present.

Depressed mood is not always a psychiatric disorder. It may also be a normal reaction to certain life events, a symptom of some medical conditions, or a side effect of some drugs or medical treatments. Depressed mood is also a primary or associated feature of certain psychiatric syndromes such as clinical depression.

Depressed mood can be the result of a number of infectious diseases, neurological conditions and physiological problems including hypoandrogenism (in men), Addison's disease, Lyme disease, multiple sclerosis, chronic pain, stroke, diabetes,cancer,sleep apnea, and disturbed circadian rhythm. It is often one of the early symptoms of hypothyroidism (reduced activity of the thyroid gland)

Life events and changes that may precipitate depressed mood include childbirth, menopause, financial difficulties, job problems, a medical diagnosis (cancer, HIV, etc.), bullying, loss of a loved one, natural disasters, social isolation, relationship troubles, jealousy, separation, and catastrophic injury.

Traumatizing events that took place in childhood can cause depression. Although childhood trauma and particularly child sex abuse is not always a factor of adulthood depression, it may create psychological pathways that can lead to depression. Research has been done in this field to demonstrate the chemical involvements explaining this phenomenon

Unequal parental treatment is also a risk factor

Psychiatric syndromes
Main article: Depressive mood disorders

A number of psychiatric syndromes feature depressed mood as a main symptom. The mood disorders are a group of disorders considered to be primary disturbances of mood. These include major depressive disorder (MDD; commonly called major depression or clinical depression) where a person has at least two weeks of depressed mood or a loss of interest or pleasure in nearly all activities; and dysthymia, a state of chronic depressed mood, the symptoms of which do not meet the severity of a major depressive episode. Another mood disorder, bipolar disorder, features one or more episodes of abnormally elevated mood, cognition and energy levels, but may also involve one or more depressive episodes.

Moderate levels of physical activity can treat depression by increasing the levels of endorphins and the neurotransmitters serotonin, dopamine, and norepinephrine. Exercise allows individuals to improve their health while building new relationships with others and bolstering the sense of community that comes with exercising as a group. Group activities can lower depression by increasing depressed individuals’ ability to interact with others. Exercise also increases individuals’ self-confidence by promoting social skills that people with depression often lack and interrupts the cycle of isolation from the general population that can further increase depression. Exercise fosters non-demanding behaviors while allowing people to socialize and identify themselves as part of the general population