Depression is a common but serious mood disorder characterized by persistent feelings of sadness, hopelessness, or loss of interest in activities once enjoyed, lasting for weeks or longer and interfering with daily functioning. It can affect thoughts, emotions, behavior, and physical health, leading to symptoms such as fatigue, changes in sleep and appetite, difficulty concentrating, irritability, and feelings of worthlessness or guilt. In some cases, individuals may experience thoughts of self-harm or suicide. Depression is influenced by a combination of genetic, biological, environmental, and psychological factors. While it is a medical condition—not a personal weakness—it is treatable through approaches such as psychotherapy, medication, lifestyle changes, and social support, with early intervention improving outcomes.