The medical term used to describe "absence of periods" is amenorrhea. Women normally do not menstruate before puberty, during pregnancy, and after menopause. If a woman does not get her period when she normally should, it may be the symptom of a treatable medical condition.
There are two types of amenorrhea: primary amenorrhea and secondary amenorrhea. Primary amenorrhea is when a young woman has not had her first period by the age of 16. Secondary amenorrhea is when a woman who has had normal menstrual cycles stops getting her monthly period for three or more months.