Depression.

Definition: A mood disorder characterized by persistent feelings of
sadness, hopelessness, and a lack of interest or pleasure in activities.

Age Group: Can affect individuals of any age, including children,
adolescents, and adults.

Symptoms: Persistent low mood, loss of interest, changes in appetite
and sleep patterns, fatigue, feelings of worthlessness, and difficulty
concentrating.

* Please note that while these descriptions provide an overview, each individual's experience with these disorders can vary, and a professional diagnosis is necessary for accurate assessment and treatment.