Merriam-Webster Collegiate Dictionary - menstruation

 noun  Date: circa 1754 a discharging of blood, secretions, and tissue debris from the uterus that recurs in nonpregnant breeding-age primate females at approximately monthly intervals and that is considered to represent a readjustment of the uterus to the nonpregnant state following proliferative changes accompanying the preceding ovulation; also period 6c