<unk>  is the absence of menstruation. Menstruation is a woman's monthly period.<unk>  is when a girl has not yet started her monthly periods, and she:
Has gone through other normal changes that occur during puberty
Is older than 15
Primary amenorrhea
Secondary amenorrhea
Chronic pelvic pain
Premenstrual syndrome
Infertility
Delayed puberty
Premature ovarian failure
Polycystic ovary syndrome