The Collaborative International Dictionary of English v.0.48:

Menstruation \Men`stru*a"tion\, n.
   The discharge of the menses; also, the state or the period of
   menstruating.
   [1913 Webster]

WordNet (r) 3.0 (2006):

menstruation
    n 1: the monthly discharge of blood from the uterus of
         nonpregnant women from puberty to menopause; "the women
         were sickly and subject to excessive menstruation"; "a
         woman does not take the gout unless her menses be stopped"
         --Hippocrates; "the semen begins to appear in males and to
         be emitted at the same time of life that the catamenia
         begin to flow in females"--Aristotle [syn: menstruation,
         menses, menstruum, catamenia, period, flow]