Depression: A mood disorder characterized by persistent sadness, loss of interest, and emotional or physical fatigue. Specialists: Psychiatrists, Psychologists, Counselors, Cognitive-Behavioral Therapists, Psychiatric Nurse Practitioners. Diagnosis: Based on patient-reported symptoms, standardized screening tools (e.g., PHQ-9), and DSM-5 criteria. Symptoms include low mood, sleep disturbances, loss of appetite, and difficulty concentrating.