Depression is a mental state of low mood and aversion to activity. It affects about 3.5% of the global population, or about 280 million people worldwide, as of 2020. Depression affects a person's thoughts, behavior, feelings, and sense of well-being. The pleasure or joy that a person gets from certain experiences is reduced, and the afflicted person often experiences a loss of motivation or interest in those activities. People with depression may experience sadness, feelings of dejection or lack of hope, difficulty in thinking and concentration, hypersomnia or insomnia, overeating or anorexia, or suicidal thoughts.

Depression can have multiple, sometimes overlapping, origins. Depression can be a normal temporary reaction to life events, such as the loss of a loved one. Additionally, depression can be a symptom of some mood disorders, such as major depressive disorder, bipolar disorder, and dysthymia. Depression is also a symptom of some physical diseases and a side effect of some drugs and medical treatments.

Adversity in childhood, such as bereavement, neglect, mental abuse, physical abuse, sexual abuse, or unequal parental treatment of siblings, can contribute to depression in adulthood. Childhood physical or sexual abuse in particular significantly correlates with the likelihood of experiencing depression over the survivor's lifetime. People who have experienced four or more adverse childhood experiences are 3.2 to 4.0 times more likely to suffer from depression. Poor housing quality, non-functionality, lack of green spaces, and exposure to noise and air pollution are linked to depressive moods, emphasizing the need for consideration in planning to prevent such outcomes. Locality has also been linked to depression and other negative moods. The rate of depression among those who reside in large urban areas is shown to be lower than those who do not. Likewise, those from smaller towns and rural areas tend to have higher rates of depression, anxiety, and psychological unwellness.

Studies have consistently shown that physicians have had the highest depression and suicide rates compared to people in many other lines of work—for suicide, 40% higher for male physicians and 130% higher for female physicians.

Life events and changes that may cause depressed mood includes, but are not limited to, childbirth, menopause, financial difficulties, unemployment, stress (such as from work, education, military service, family, living conditions, marriage, etc.), a medical diagnosis (cancer, HIV, diabetes, etc.), bullying, loss of a loved one, natural disasters, social isolation, rape, relationship troubles, jealousy, separation, or catastrophic injury. Similar depressive symptoms are associated with survivor's guilt. Adolescents may be especially prone to experiencing a depressed mood following social rejection, peer pressure, or bullying.

A body of high-quality longitudinal research has linked adverse working to increased depressive symptoms and disorders. Workplace stressors that increase depression risk include excessive workloads, little autonomy, an unfavorable effort-reward imbalance, and workplace bullying.

Depression in childhood and adolescence is similar to adult major depressive disorder, although young sufferers may exhibit increased irritability or behavioral dyscontrol instead of the more common sad, empty, or hopeless feelings seen with adults. Children who are under stress, experiencing loss, or have other underlying disorders are at a higher risk for depression. Childhood depression is often comorbid with mental disorders outside of other mood disorders; most commonly anxiety disorder and conduct disorder. Depression also tends to run in families.

Depression is associated with low extraversion, and people who have high levels of neuroticism are more likely to experience depressive symptoms and are more likely to receive a diagnosis of a depressive disorder. Additionally, depression is associated with low conscientiousness. Some factors that may arise from low conscientiousness include disorganization and dissatisfaction with life. Individuals may be more exposed to stress and depression as a result of these factors.