The document discusses amenorrhea, which is the absence or abnormal cessation of menstruation. It defines physiological and pathological amenorrhea and describes the classification and etiology of primary and secondary amenorrhea. Primary amenorrhea refers to the absence of menstruation by age 16, while secondary amenorrhea is the cessation of established menses for over 6 months. Common causes include hypothalamic-pituitary-ovarian dysfunction, congenital abnormalities, pregnancy, and thyroid or weight issues.