Major Depressive Disorder is a mood disorder characterized by symptoms like depressed mood, feelings of hopelessness, decreased interest in activities, and social withdrawal. It often occurs concurrently with other psychiatric diagnoses and is influenced by biological, genetic, environmental, and psychological risk factors. Signs include loss of pleasure or interest, changes in appetite, sleep, and energy, feelings of worthlessness, difficulty concentrating, and suicidal thoughts. Nursing diagnoses for those with Major Depressive Disorder focus on safety, communication, decision making, roles, and mood. Treatment involves medication, psychotherapy, social support, and ensuring a safe environment.