Amenorrhea is the absence of menstruation in women of reproductive age. It can be classified as primary amenorrhea, where menstruation has not begun by age 15, or secondary amenorrhea, which refers to the cessation of periods for three consecutive cycles or more in someone who previously had regular menstruation. Causes can range from hormonal imbalances, excessive exercise, stress, and nutritional deficiencies to underlying medical conditions. Treatment depends on the underlying cause and may involve lifestyle changes, hormonal therapy, or addressing specific health issues.