What is amenorrhea and what causes it?

Amenorrhea can be primary or secondary. Primary amenorrhea occurs when a woman has not had her first menstrual period (menarche) by age 15 or 16. This condition is also called delayed menarche. It is most often due to late puberty. This is fairly common in teenage girls who are very thin or very athletic. 
Secondary amenorrhea occurs when a woman has experienced menstrual periods, but stops menstruating for three or more consecutive months. Secondary amenorrhea is expected in pregnancy, breast feeding, and menopause. Other common causes include emotional or physical stress, rapid weight loss, frequent strenuous exercise, hormonal birth control methods, and polycystic ovary syndrome (PCOS).