Webster's English Dictionary

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menstruation

men.stru.a.tion \.men(t)-str*-'wa--sh*n, men-'stra--\ n : a discharging of 
   blood, secretions, and tissue debris from the uterus th at recurs in 
   nonpregnant breeding-age primate females at approximately monthly intervals 
   and that is usu. held to represent a readjustment of the uterus to the 
   nonpregnant state following proliferative changes accompanying the 
   preceding ovulation; also : PERIOD