Amenorrhea is the medical term for the absence of menstrual periods in women of reproductive age. It can be classified as primary, when a girl has not started her periods by age 15, or secondary, when a woman who has previously menstruated stops for three months or more. Several factors can contribute to amenorrhea, including hormonal imbalances, excessive exercise, low body weight, stress, and certain medical conditions. It is important for individuals experiencing amenorrhea to consult a healthcare provider for proper diagnosis and treatment options.