Depression is a mental illness characterized by feelings of sadness and despair that can affect thinking, behavior, and physical well-being. It has several types including major depression. Causes may include genetic, brain, environmental, psychological, and social factors or other conditions. Symptoms include feelings of emptiness, guilt, fatigue, changes in appetite, and thoughts of suicide. Treatment involves support, psychotherapy like CBT, medication, exercise, diet, light therapy, and ECT in severe cases. Nursing management focuses on assessment, diagnoses related to self-care deficits, nutrition, sleep, and risk of harm.