Depression, also known as major depressive disorder, is a common and serious medical illness that negatively affects how a person feels, thinks, and acts. It is characterized by persistent feelings of sadness and a loss of interest in activities once enjoyed. Depression can lead to various emotional and physical problems and can decrease a person's ability to function at work and at home.

Symptoms of depression can be emotional, physical, or cognitive. Emotionally, individuals may experience persistent sadness, anxiety, or a feeling of emptiness. They might also feel hopeless, pessimistic, irritable, guilty, worthless, or helpless. Physically, depression can lead to decreased energy or fatigue, changes in appetite or weight, sleep disturbances, and unexplained aches or pains. Cognitively, those affected may have difficulty concentrating, remembering things, or making decisions. In severe cases, they may have thoughts of death or suicide or even attempt suicide.

The causes of depression are complex and involve a combination of genetic, biological, environmental, and psychological factors. A family history of depression can increase one's risk, as can chemical imbalances in the brain. Stressful life events such as the death of a loved one, divorce, financial problems, or job loss can also trigger depression. Additionally, negative thinking patterns and low self-esteem can contribute to the development of the disorder.